Operation Tracer: The plan to bury six men alive inside The Rock of Gibraltar

Rumour had it a secret sealed-off chamber had been prepared to leave behind six servicemen – Forces News got to take a look inside.

On Boxing Day 25 years ago, a mystery that had been tormenting Gibraltar for decades was solved.

Rumour had it that somewhere in The Rock, a secret sealed-off chamber had been prepared to leave behind six servicemen.

Why? So they could spy on the Germans should they invade and occupy.

As Germany marched on Europe, the invasion and occupation of Gibraltar became an increasing threat and the British needed a plan – Operation Tracer. 

It meant burying six soldiers alive inside the rock, creating a secret chamber inside The Rock where they could hide, potentially for years.

Their mission was simple: to silently observe the occupying enemy and feed intelligence back to London by radio transmissions in the dead of night. 

Watch: Gibraltar – The Rock's WW2 and gambling secrets.

The Germans never did capture Gibraltar and the secret chamber, which became known as Stay Behind Cave, was never needed.

However, many believed the prepared chamber still existed somewhere in the rock.

Nobody knew where it was and nobody knew where to start looking for it.

However, on Boxing Day in 1996, a group of cavers found what Gibraltar had been searching for.

Watch: Gibraltar – what's inside The Rock?

The cave was fitted with a main room, where the personnel would have slept and stored food and equipment.

The floor was covered in cork tiles and horse dung was mixed into the plaster on the walls to deaden noise.

There was also a radio room, ventilation shaft, two toilets (in case one broke) and an aerial that, situated in a pipe, could be pushed out at night to transmit messages before being brought back into the cave and out of sight.

The mystery is only partly solved, however.

From Stay Behind Cave, there is no way of seeing what is happening on the North of Gibraltar – so the search remains to see if there is a second secret spy chamber on the island.