It's like something out of a James Bond movie.
And it's not surprising, as 007 writer Ian Fleming himself helped design this highly classified World War II military project.
It was back in 1940 that plans for what became known as “Stay Behind Cave” were set in motion. At the time Germany was planning to capture Gibraltar from the British in a plan code-named Operation Felix.
So serious was the threat that a top intelligence officer of the British Navy - Rear Admiral John Henry Godfrey - decided to build a covert observation post in Gibraltar that would remain operational even if the Rock fell to the Axis powers.
From this base the movements of enemy ships would be reported to GCHQ in the UK with clandestine radio communication.
Officially known as Operation Tracer, construction of the listening post began in earnest in 1941 in what is now the Upper Rock Nature Reserve on the southern side of Gibraltar.
The plan was so tight-lipped that Godfrey held meetings with his consultants at his private residence in Mayfair rather than at Whitehall in London.
Among those consultants who helped with the detailed planning for Operation Tracer was a young Ian Fleming, at the time a naval Volunteer Reserve Officer, and one of Godfrey's assistants.
Those trusted with the building of the underground facility were blindfolded to and from the construction site each night under the cover of darkness, and after its construction, they were posted back to the UK and not allowed overseas until the end of the war.
By the summer of 1942 the cave was ready - and a team of six men were selected for the operation, an executive officer as leader, two doctors and three wireless operators.
All six men had volunteered to be sealed inside the cave should Gibraltar fall to the Germans. The initial plan was that they would remain sealed within the facility for a year - but then this was increased to seven years.
All were given cover jobs in Gibraltar during the day, but at night they were transported up to the cave to be trained for a day they hoped would never come.
The plan was that the team could be in the cave within the hour should Gibraltar wake up to an unexpected invasion.
Inside was a living room, three bunk beds, a communications room, a bathroom and two observation points looking over the Bay of Gibraltar on the West and the Mediterranean Sea in the East. Provisions for seven years were also stored within the complex.
What's more, the cave floor was covered in cork tiles to reduce the sound in case any enemy soldiers came close.
In addition, a bicycle with a leather strap instead of a chain (to keep the noise down) was put in the cave to provide electricity for the wireless radio and to keep the men fit.
There was no way out and if anyone died within the chamber they were to be embalmed and buried within a small soil-filled spot close to the entrance - giving a rather sinister feel to the aptly titled Stay Behind Cave.
An eery feature is that you can still see the makeshift grave all these decades later.
Of course, the Germans never invaded Gibraltar and the cave was all but forgotten.
Intelligence chiefs ordered that the provisions in the complex be removed and the cave was sealed up.
Rumours of a secret complex circulated for decades in Gibraltar, until its accidental discovery on Boxing Day in 1997 by the Gibraltar Caving Group.
Phil Smith, 55, a senior guide and site manager at Stay Behind Cave, which is managed by the Gibraltar Museum, told SUR in English:
“They had been looking for it for years and whilst passing through a known tunnel, luckily stopped to have their sandwiches by the cave entrance on that fateful day in 1997. They felt a wind coming through the corrugated iron lining of the tunnel where no wind should have been.
“On removing a panel, they saw shuttered concrete instead of the expected natural limestone and a little further down found the cave entrance itself.
“It was more or less as it had been left in 1942.”
The authenticity of the site was confirmed by one of the builders in 1998, and a decade later by one of the doctors - Dr Bruce Cooper.
That doctor, the last surviving member of the Tracer team, died in 2010. Sworn to secrecy, he hadn't even told his wife or children about his involvement in the secret complex.
Nowadays the exact location of Stay Behind Cave is still kept secret in order to preserve the complex - and the facility is under lock and key.
However, the Gibraltar Museum organises around 30 guided tours a year and plans are now afoot to make the cave more accessible - in order to increase the number of visitors.
Phil added: “I think the cave fascinates people as it's such an incredible story - in this case the truth really is stranger than fiction.
“All the secrecy does sound like something out of a James Bond film and of course that's because of Ian Fleming. So all these villains and lairs - this is probably where he got his ideas.”
In yet another twist to the tale, there are rumours that a second Stay Behind Cave exists on the Rock.
“The known cave does not overlook the runway and surely detailed information of aircraft movements would be just as important as ship movements,” argued Phil.
“Someone claims to have seen a plan for a Gibraltar Operation Tracer Cave with a different layout to the one we know about. Another story is that one of the construction party who worked on the project was shown the layout for the known cave and said he didn't recognise it. Whatever the truth, having eyes on the runway would seem to me vital to the success of the operation.”