The tunnel system known as Lord Airey’s Shelter is still owned by the Ministry of Defence, except the Tracer cave which is under the guardianship of the Gibraltar Museum. Access is tightly controlled and limited to around 20 people per year, with guided tours arranged through the museum for a fee. As soon as I had booked flights, I contacted the museum and arranged a tour. The images below were taken on this approximately 90-minute tour. Thanks to the museum for permitting me to take photographs.

You can view my high-resolution photographs on Flickr or follow through a virtual tour.

This is the time to give some context to the Tracer tunnel as it lies in 2024. The tunnel now known as Tarcer Cave was discovered in Christmas 1997 by the then-named Gibraltar Caving Group. Their remarkable discovery of the cave was reportedly by accident when they were passing through the tunnel and one of the members felt an uncharacteristic breeze passing through from behind the corrugated iron lining. Their story expands into saying they peeled back some damaged corrugated iron and discovered a bricked-up entrance. However, I think the discovery was no accident. In a 1997 documentary hosted on YouTube (link), they tell of some details around the theme of a draught, but had to remove some corrugated iron and brickwork to reveal it. Their “break-in” to the cave, in their own words, was undertaken during Christmas 1997, a period when I imagine they would not expect to be disturbed, seems to me very targeted and based on information revealed in the National Archive document. Based on the damage incurred to the access tunnel, shared with Lord Airey’s Shelter, they were discovered to reveal the Tracer chamber during a series of visits that year. In their defence, if they had not done so we would still not know of the presence of the tunnels today, but with hindsight, I also think their approach seemed reckless and haphazard.

Now that Tracer has been uncovered, it is under the protectorship of the Gibraltar Museum. While they do permit guided access, this is only to a relatively limited number of visitors each year. There have also been no attempts at conservation undertaken, and the tunnel system continues to deteriorate as moisture settles deeper into the fabric of the chamber. Some items, such as a leather strap reported to have been used as an alternative to a chain on the bicycle generator, have been removed to the museum for secure storage. I am also not aware of any archaeological survey having been undertaken in the tunnel to piece together what evidence remains.

Plan

The following plan is not to scale but is based on my experience inside the cave and comparisons with other plans people have drawn. It isn’t to scale but is representative of the layout of the tunnel system.

A – Camoflagued entrance with bricks and corrugated iron. Internal second aperture covered by tarred paper. Jump to section
B – Permanently sealed entrance backfilled with shuttered concrete. Jump to section
C – Rock cut entrance hall. Jump to section
D – Soil floor. Collapsed door to the main chamber. Jump to section
E – Main chamber with cork tile floor and plastered walls and ceilings. Jump to section
F – Twin macerating toilets, Elsan branded ceramic bowls. Jump to section
G – Room with remains of electrical and radio fittings. Jump to section
Stairway. Jump to section
Ventilation. Jump to section
H – Eastern observation post. Large aperture. Jump to section
I – Possible radio corridor. Jump to section
J – Western observation post. Small aperture. Jump to section

Tracer entrance

This is the view of the main entrance into the Tracer tunnel (B) from the corridor joining Lord Airey’s Shelter to the outside road (A). The debris littering the floor is from the attempts to gain access to the tunnel; as careful as the Gibraltar Caving Group said they were, there was a considerable amount of corrugated iron and bricks removed. The initial discovery 25 years ago may have prompted further explorers exacerbating the damage.

In this comparison of two images showing the internal entrance in late 1997 or early 1998 (taken by Jim Crone and available on the Operation Tracer Facebook page) compared with how the entrance appeared in 2023 during my visit, it’s clear to see how much of the biological matter (the wood in particular) has degraded.

We can also see the initial hole in the wall made by the Gibraltar Caving Group to gain access, and how this has been expanded over the years to facilitate easier access.

Two other points to note in this image are on the right side; parts of this tunnel system are natural crevices and caves that have been modified for use, but also note the rolled-up piece of tarred paper. This may have once been used as a curtain across the second entrance we can see with the wooden lintel.

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Sealed entrance

One feature that has often perplexed people is the presence of an apparent blocked entrance in the entrance chamber. I believe this would have been the original entrance to the shelter before it was acquired for Tracer. It is similar to the upper entrance that appears to have been blocked and used as an observation post.

A – Sealed entrance with wooden shuttering and poured concrete. The fill hasn’t reached the full height of the doorway.
B – Construction materials, bricks and wood.
C – Metal tins of cement, likely used during construction and never removed. Also thought to have been stored for use in cementing up anyone who died.

There is one more curious feature about this part of the cave: this shallow recess. Its location is at about chest height to the left of the sealed doorway, but it appears to have been cut AFTER the forming of the concrete wall as it is not only excavated into the rock but also the concrete. With no apparent purpose, part of me wonders if this was a booby trap or fougasse? Certainly worth further examination and discussion (contact me at the bottom of this article).

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Rock-cut entrance chamber

This entrance chamber has been cut from the rock and contains two entrances, one exit into the main Tracer chamber as well as a natural fissure. Of the two entrances, one is the blocked entrance detailed in B, which appears full height and width. The second is the Tracer entrance detailed in A, which is smaller and more crude, likely having been cut to serve Tracer once the main entrance was sealed.

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Soil floor and main chamber entrance

Outside the main chamber is thought to be where the team would dispose of bodies should team members die during the operation. While the location is not confirmed, the summary text discusses “the disposal by embalming and cementing up of anyone who died.” The soil in this area of the tunnel is speculated to have facilitated the silent digging of a grave. Buring your dead under the main entrance seems a bit macabre to me, and I would hypothesise that the soil may indeed have deadened footsteps if the team exited the cave into the entrance hall, but that the embalmed bodies would have been stored and possibly encased in concrete elsewhere in the chamber.

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Main chamber

The team is thought to have lived in the main chamber during the operation. It is finished to a higher standard than many of the Gibraltar tunnels and shelters. The solid roof and ceiling, as well as the plastered walls, are almost unique to such a chamber. The floor has also been covered in cork tiles, which may have helped silence footsteps but also added warmth to the otherwise cold concrete floor. There is no evidence of cork tiles anywhere else in the tunnel.

I find a few features, or lack of them, remarkable about the main chamber. I will raise these now and as you look through the images, you can decide for yourself.

There is no ventilation. None. With both doors closed, there is no way for air to circulate, and with humidity known to be high in the tunnels of Gibraltar, I find it astonishing anyone would have lived in this room.
There are also no signs or any fixtures or fittings. If this room was indeed fitted out for a team of 6 people to live for upwards of a year, I would have expected to see witness marks on the walls from furniture or even remains of low-value items not removed when the operation was closed down.
I also appreciate that electricity was at a premium, but there is no evidence of any light fittings, cables or switches. These are present in what is known as the radio room, but none in the main chamber. With the lack of ventilation, I would doubt oil or acetylene lamps could have been used, leaving only electricity as the clean source.
There is also no sign of anywhere for water to be heated or cooking to be done; albeit heating food to reduce possible smells from terminating from the chamber. A pressurised alcohol stove could have been used in the ventilated stairwell, such as those issued as field cookers.
With the sewage from the toilets being drained into a crevice in the corner of the room, the potential for an unhealthy stench was high.

A – This aperture leads to the 10,000-gallon water tank or cistern. The tank is known as a sectional Braithwaite tank, common in use by the British military for over a hundred years. The tank is reportedly topped up by a natural drain in the rock above and feeds two taps; one in Tracer cave and one in Lord Airey’s Shelter.
B – The tap from the water tank is in the corner of the room and drains into a sump. This may have been a feature of the original tunnel system adopted for use with Tracer.
C – The doorway to/from the access tunnel. The remains of a wooden door are on the internal floor, the frame also having fallen out.
D – A drain has been fitted to the corner of the room, shared with the waste pipe from the toilets. This appears to be a modification to the original tunnel structure.
E – The unusual patch of plastering at the north end of the chamber was broken through by the Gibraltar Caving Group in an attempt to discover what was on the other side of the wall. Nothing was found other than bedrock.
F – The waste pipe from the toilets appears to have been fitted after the chamber had been plastered and is contemporary with drain D.
G – Concrete stairs leading to the upper tunnels and toilets. I believe these were part of the original construction of the tunnel and not specific to the Tracer fit-out. The remains of a wooden door are on the internal floor, the frame also having fallen out.
H – The toilet room contains two Elsan-branded dry toilets with a macerating mechanism below. The mulched waste would have drained through the pipe and F and into drain D.
I – This is where Dennis Woods signed his name in 1941.

Image E – The brick pattern at the north end of the chamber

Dennis Woods visited the cave in 1998 and confirmed that he worked on the construction of this cave. Mr Woods had been responsible for the plastering of this room, and he confirmed that the entire room, including the ceiling had been plastered to provide further sound insulation.¹ The plaster was signed and dated 1941. This is a very early date for confirmation of the cave to be used for Operation Tracer. He may have signed it not knowing the potential use, but as visitors such as intelligence officers and wireless telegraph operators started to visit conducting reconnaissance and radio tests, it may have become clear the cave was to be used for something special. I believe that due to the sensitive nature of the operation, men involved in the construction would not have been aware of Operation Tracer and the future role it may have played.

Image I – Dennis Woods signed his name here in 1941 after apparently plastering the chamber.

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Stairway

The chronology of construction is difficult to ascertain, especially from a short and restrictive tour. By capturing as many images as I could I have been able to carry out some analysis in a slower time and from the comfort of my desktop. In the upper areas of the tunnel, there are two distinct construction methods. The first is the corrugated iron-lined tunnel, as seen elsewhere in Lord Airey’s tunnel and others. The second method, used in the tunnel leading to the east observation post is a concrete-lined tunnel rising either side of a concrete staircase. In the left gutter are two pipes laid in the concrete, and at the top, there is a half-height wall, and quite noticeably no platform. The stairs stop and are blocked by the wall. The aperture is large enough for a person to climb outside but has been camouflaged using cement to blend in with the rock and there appears to have been a wooden frame with further camouflage that could have blocked the aperture when not in use.

Looking at the construction methods and finish of other tunnels in the area, it seems a fair conclusion that they were constructed at a similar time and there is nothing remarkable about them.

In this early photograph taken soon after the discovery of Tracer, there is a fitted piece of wood covering the pipe on the left of the stairwell. This piece doesn’t appear to be in the tunnel any more or has broken up and I couldn’t identify it. Its purpose is unclear but it is a very intentional inclusion in the design of the tunnel.

The image is believed to have been taken by Jim Crone in the late 1990s and downloaded from Facebook in October 2023.

One of the two pipes running alongside the east stairway is this. It appears to be an asbestos-cement pipe. It is also possible there is a lead covering to this pipe, giving it some flexibility. Approximately 200m from the end of the pipe there is a hole in the side wall.

The purpose of this pipe is not confirmed, but it has been suggested that it was to house the radio antenna. I have discounted this theory due to the practicality of housing the antenna in the pipe and having to feed it from the stairwell until it protrudes from the side of the cliff (assume this pipe opens on the exterior of the tunnel). If it is indeed made from lead and asbestos-cement, it would be a good insulator against RF energy. It would take a poorly designed radio system to protect the occupants of the building against emissions from the antenna.

The pipe is exposed for a considerable length after leaving the concrete conduit alongside the stairs.

As well as the pipes along the stairs, there are signs that there may have been overhead fixtures:

There is evidence of overhead brackets on both sides of the corridor. These are holes drilled into the rock or concrete then filled with a wooden plug and bolted into. The ventilation duct on the right side has fallen to the ground, but it’s not clear what was mounted between points A and B.

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Toilets

Two toilets were provided in the Tracer tunnel, more likely for redundancy than comfort. They are non-flushing, with waste being macerated in the chamber below. Both sit high on a concrete bench and a foul air pipe for each toilet evacuates stench into the open space above the room. There are remains of a tarred paper and wire mesh ceiling.

The shape of the room is unusual and gives the appearance that it was constructed after the main chamber was completed. Further suggesting modifications after the complex for Lord Airey’s shelter was repurposed.

The porcelain toilet bowls were made by Elsan, a company well known for their chemical toilets during the war. I contacted the modern-day company asking if they might have more details on these particular bowls and was told: “Elsan didn’t make porcelain toilets, but it was acquired by a traditional plumbing specialist, Sanitas, and they may have used the Elsan brand to market some of their porcelain toilets.”

One of the bowls is broken and this has been put down to vandalism. It is possible that a small rockfall from the cave roof caused this naturally unless evidence to the contrary is suggested.

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Radio room

The absolute heart of the operation, the radio room is the only room we have evidence of use. When the caving group first explored the tunnel system they discovered several artefacts related to electricity and communications equipment. This included ceramic and glass insulators often used to isolate live wire antennas during transmission, a knife edge switch to turn power on/off and the remains of a hand crank likely used as part of the rudimentary ventilation system. The images below are screenshots from the Tracer documentary by Clive Finlayson on YouTube.

Much like the small toilet room, the radio room had a ceiling made from tarred paper and wire mesh laid over timber planks. Inside the room were also remains of a bench, a shelf, and electrics including a light fitting, wiring and a bakelite light switch. This room was likely used for some of the communication tests.

Electrical light switch (left) and a light bulb fitting (right) on the floor of the radio room among the collapsed ceiling debris.

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Forced air ventilation

Gibraltar is known for the Lavanter, a strong, warm and damp wind from the east. The larger aperture in Tracer is the one to the east, and nearest the stairway, radio room and toilets. As strong as the wind may be, once the air pressure inside the tunnel system is equal to that outside, unless there is an escape fo the air, it will not blow through the tunnel. It is comparable to blowing across an empty wine bottle, as opposed to blowing through a straw. And with such a small easy aperture, and the main chamber entrances closed up, there was only one way for air to enter and exit the tunnel. Some form of forced air ventilation would be required, and there may be evidence of that.

There has been some discussion about the use of a bike for ventilation in the cave, and indeed there are remains of a belt-driven fan on the roof of the radio room, as well as remains of intact ducting now on the floor. Examining the fan in more detail, it appears to be made from cast aluminium and has four blades. There is a pully on the drive spindle, suggesting it was belt-driven. The fan is centrally mounted in a tubular steel frame, which in turn has vibration dampers at the points where the frame is mounted to the housing. There is a handle placed on top of the fan but it does not appear to be associated with it.

Also to note is the remains of a modified bike frame in the same pile of debris, this appears to have a sprocket suggesting this was chain-driven. A leather strap has been reported as being recovered from the tunnel and is in the museum for safe storage. This has been suggested as a low-noise belt drive.

The duct which appears to be the intake or exhaust stops at the intersection of the two staircases leading to the east post and heading up to the west post. The only reason I can attribute to this is that there was a sufficient flow of air between the east and west posts to provide a change of air if the fan was turning.

Having a hand-powered ventilation system would be essential to prevent the building up of moisture on the electrical equipment, and also to potentially remove stale air from the toilets. But it could also have been used to extract steam or odours from a cooking area. There has not yet been any evidence for this, but it would be almost essential for the team to have the facility to heat food and water,

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Observation post (east)

As mentioned in the section about the stairwell, the stairs leading up to the east observation post end abruptly. If this section had been designed as an observation post I would expect a platform or cubicle would have been constructed here, perhaps with some way to isolate the observer from the stairway. There is evidence that suggests a frame and curtain hung behind the west observation post, but there is no such evidence here. The group that discovered the tunnel system identified a ledge outside this post that was large enough and secure enough for men to stand or sit on. It has been suggested that this may have given the group some sunlight and fresh air. If this was the case, then I would have expected to see an easier way of exiting the tunnel for them.

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Radio corridor

None of the retrospective articles, reports or reviews of Tracer refer to the radio corridor. In the contemporary summary report (line 19 for reference) it states that “priority of construction was being given to the passage-section designed to contain the wireless installation.” This was reported by Commander Birley SO(I) Gibraltar on 27 December 1941. He went on to state “It was hoped to complete this by 15th February (1942) and to undertake experiments on communications without delay.”

What has confused me is the reference later on to a space “4 feet wide, 2 feet high and 18 inches deep.” These dimensions do not appear to correlate to any space within the recognised Tracer tunnel. Were these the dimensions for a shelf on which the radios would be installed? However, in the upper corridor of the system, the end of which is the western observation post, are some features that make me wonder if this was the passage section originally designed to contain the wireless equipment.

The corridor above has many similar features to tunnels throughout the rest of Gibraltar. I believe this was originally part of the system constructed which was called Lord Airey’s Shelter. There have been some modifications that may have been in support of Tracer. I have labelled these A-C.

A – A series of holes have been drilled into the concrete walls of this tunnel section. They have the appearance of holes drilled by the tunnelling companies using industrial drilling equipment and not more domestic rotating drills. The comparison image to the left shows the similarities between a rock-cut hole (left) and the holes in the concrete here (right). Remains of steel and wood inside some of these holes suggest they were used to mount hardware; perhaps a bench or shelves. They do not appear anywhere else along this tunnel but are present along both sides.
B – A copper strap is protruding from the concrete behind the corrugated iron. To optimise radio reception a reliable earth is required. Copper earthing straps like this are common in radio installations. I did not see this anywhere else in the tunnel.
C – There are remains of timber in this tunnel. Photographs taken soon after the tunnel was discovered indicate these may have been a frame, perhaps to block the light while the observation post was in use.

Shown here is another image from the Operation Tracer Facebook page and this time it shows the radio corridor frame intact. The wooden frame appears to be profiled to fit across the tunnel, perhaps to act as a screen to block light or to give some segregation to the person who may have been on duty or operating the radio.

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Observation post (west)

The observation post to the west is the main, well-camouflaged post from where observations on shipping and the docks would have been carried out. At one time it may have been an upper entrance to Lord Airey’s Shelter system, but was later blocked up and a small aperture excavated in the concrete. A step has been added and does not appear to have been part of the original construction of this entrance. The entrance also narrows in width from the main tunnel, another sign it has been modified from previous use.

Rock and debris have been used to pack out the end of the tunnel when it was blocked off and sealed over, these can be seen above and around the shuttered concrete. The concrete has been reinforced for strength and the reinforcing iron can be seen protruding and corroding through the profiled concrete observation slit.

The tunnel leading to the observation post I have called the radio corridor

In the final 360-image from the Tracer tunnel below, notice how the observation slit, or loophole, has been angled to favour viewing towards the harbour. Evidence if there was needed that this was the main objective of the team. The wall has also been carved away to the left of the position, and this is likely to help accommodate an observer who may be using binoculars or a telescope and needed to be further back from the slit. Without this, the observer’s head would be right up against the wall in a very awkward position.

There is very little in terms of graffiti or other telltale signs of occupation in the tunnel system, other than D Woods leaving his name beside the door. However, when reviewing my photographs of the tunnel I discovered some very subtle markings on the side wall of the west observation post.

It appears as if there is a closed horizontal rectangle, with a pair of vertical lines extending from the centre bottom of it. Inside the main rectangle appear to be two rows of text. The marks, albeit slight, appear to have been made in soft concrete, perhaps after the formwork had been removed, suggesting they were made at the time of construction.

I have altered the contrast of the image, left, and it is difficult to make out, but they could be initials on the top row (HIO?) and a date on the lower row (9.2.42?). This will take further investigation of the site to determine the contents fully.

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Closing remarks

There is still a lot to learn about Tracer, but I have done my best to carefully consider the evidence presented to me, and have also hopefully given you some matters to consider. I will also share some of my conclusions, reached due to lack of evidence or implausibility.

I don’t believe the tunnel system was ever fully furnished and stocked, ready to deploy at a moment’s notice.
The cave was very much an open secret in Gibraltar for generations, there must have been a wider knowledge of it than we appreciate.
On days where a heavy Lavanter shrouds the Rock, the post would have been blind to all ship movements.
The main chamber was unsuitable for living with 6 men and a year’s worth of supplies. It may have been planned for storage use, and the men may have slept in the upper corridor near the west observation slit.
Embalming and burying the dead must have been a real possibility, but I do not think this would have taken place in front of the main chamber door. The report states they were embalmed and encased in concrete, it is possible coffins were available for this purpose.
Perhaps the only occupied part of the cave as we find it was the radio room where the initial and final communication tests were carried out.

With so much in the media across the previous 25 years about Operation Tracer, I will look at some of the wider statements and apply some rigour to them.

Statements	Nick’s Notes
The bicycle additionally set in motion the ventilation system.² This was to have been used to drive a ventilation system and to generate electricity to power the radio equipment³	There is evidence there was a fan attached to a belt drive, as well as a short length of ventilation duct. It appears as if the fan, generator dynamo or both may have been hand-driven. For a fan to be effective at ventilating parts of the tunnel it would have needed to be electrically driven, and to the best of my knowledge only one set of handles has been found, so perhaps an electric motor did power the fans.
To reduce sound transmission, the entire main chamber was plastered and the floor was covered with cork tiles.⁴	Possible, but I think more to reduce dampness and keep the climate a little more controlled. There are no signs the large chamber was for living, I believe it would have contained the supplies. The stage the Tracer tunnel would be occupied would be when the Rock was under attack and likely to be overrun. There are several 9.2-inch heavy guns along the ridge at Lord Airey’s and O’Hara’s batteries, as well as the nearby Breakneck battery. With these guns firing, as well as taking incoming fire from the invading troops, there would have been considerable vibration and damage being inflicted on the upper rock. The steel roof beams and cork tiles may have helped protect and isolate the contents and occupants of the cave from this.
Former telegraph operator Dennis Woods in September 1998 said that he played an important role in the construction of this facility. He also stated that there were other tracker (sic) groups in Gibraltar, but his group was the main one. Woods suggested that the Tracer units worked during the Suez crisis.⁵ In September 1998, former telegraph operator Dennis Woods admitted that he played an important role in the construction of this facility.⁶ Mr Woods had been responsible for the plastering of this room, and he confirmed that the entire room, including the ceiling had been plastered to provide further sound insulation.⁷	Mr Woods is described as a telegraph operator and a plasterer. I am sure he may have held both skill sets, but doubt he would have been employed in Gibraltar as both. He is always referred to as a civilian, and I am uncertain of the number of civilians permitted on Gibraltar during the war, especially those brought in from England. I believe Mr Woods is now deceased, but I wonder if he was one of Gambier-Parry’s men, trained in a civilian occupation but able to communicate as a telegraphist when needed. It may explain his presence in Gibraltar and his knowledge of the Tracer operation, but this is pure speculation, verging on fantastical.
On a strong Levanter day in late 1997, the Gibraltar Caving Group came across a strong gust of wind in a tunnel. Further meticulous searching led them to a system of chambers, which they soon recognised as being the lost ‘Stay-Behind Cave’.⁸	I believe the caving group had worked out from the released archive materials where to search. There does not seem anything meticulous about their search. To their admission, they broke into the cave when they believed they were on the right track.
(Dr Bruce) Cooper would enter the Rock Hotel through the front door dressed as a surgeon-lieutenant RNVR and leave by the back door as an Army sergeant, then disappear into a tunnel.⁹	I think this may have a little dramatic licence, but indeed there was access to the military tunnels from the rear of the Rock Hotel. The entrance would take people into the Brigade Headquarters system. This was incidentally nowhere near Tracer, and if Dr Cooper was entering the system using this door dressed as a Sergeant it is unlikely he was going to the Tracer cave. It is more likely that he was fed up with soldiers saluting him as a Navy officer so put on a khaki battledress to make his way to the underground hospital. There is a press photograph of Dr Cooper that may have been taken in the underground hospital, putting some credibility to this story.
(…) The plan was later modified to accommodate six people instead with enough provisions to last up to seven years if necessary.¹⁰	This was perhaps part of an oral history given by Dr Cooper when he was interviewed, but I have not found anything about 7 years in the records. The additional rations for a start to sustain 6 men for 7 years on a reasonable diet would be immense.
Men involved in the construction of the Tracer tunnel were removed from Gibraltar as soon as they were finished so they didn’t spill the secrets if captured.¹¹Various sources[.mfn]	If Gibraltar was ever to enter siege conditions, every member of the population would have to be justified given the resources it would take to keep them alive. The majority of the civilian population had been evacuated, and daily workers often came across the border from Spain to fulfil labour jobs around the Rock. Military personnel not involved in the defence or engineering tasks are likely to be posted away when their task is complete. I don’t think it’s unusual that people may have been posted elsewhere when Tracer was complete, not unless it was apart from the formation or group they deployed with.

1
Gibraltar Heritage, Volume 1, Issue 1, 2002. Accessed via the Wayback Machine internet archive.
2
Ruslan Budnik, War History Online, 20 March 2019
3
Gibraltar Heritage, Volume 1, Issue 1, 2002. Accessed via the Wayback Machine internet archive.
4
Ruslan Budnik, War History Online, 20 March 2019
5
Ruslan Budnik, War History Online, 20 March 2019
6
https://www.ministryforheritage.gi/heritage-and-antiquities/stay-behind-cave-complex-1072
7
Gibraltar Heritage, Volume 1, Issue 1, 2002. Accessed via the Wayback Machine internet archive.
8
Gibraltar Heritage, Volume 1, Issue 1, 2002. Accessed via the Wayback Machine internet archive.
9
Obituary of Surgeon Lieutenant-Commander Bruce Cooper, The Daily Telegraph 4 January.
10
https://www.ministryforheritage.gi/heritage-and-antiquities/stay-behind-cave-complex-1072
11

Operation Tracer, Part 2 – Exploring the tunnel

Plan

Tracer entrance

Sealed entrance

Rock-cut entrance chamber

Soil floor and main chamber entrance

Main chamber

Stairway

Toilets

Radio room

Forced air ventilation

Observation post (east)

Radio corridor

Observation post (west)

Closing remarks