Operation Tracer was a highly classified World War II military operation based in Gibraltar, then a British colony and military base. The impetus for the plan was the 1940 scheme by Germany to capture Gibraltar, code-named Operation Felix. Operation Tracer was the brainchild of Rear Admiral John Henry Godfrey, the Director of the Naval Intelligence Division of the British Admiralty. In 1941, he decided to establish a covert observation post at Gibraltar that would remain operational even if Gibraltar fell to the enemy. From the facility, the movements of enemy vessels would be reported back to the United Kingdom. Godfrey requested the assistance of several distinguished consultants to bring the plan to fruition. The plan was so top secret that Godfrey held meetings with his consultants at his private residence rather than at Whitehall. The decision was made to construct the facility using the existing tunnel system for Lord Airey's Shelter, the underground military shelter just north of Lord Airey's Battery. The artillery battery was located at the upper ridge of the Rock of Gibraltar, near the southern end of what is now the Upper Rock Nature Reserve.Construction of the underground facility began in late 1941, and was completed by late summer 1942. The chambers served as a dual observation post, with an observation slit overlooking the Bay of Gibraltar and a larger aperture over the Mediterranean Sea. Six men were selected for the operation, an executive officer as leader, two physicians, and three wireless operators. All six men had volunteered to be sealed inside the cave should Gibraltar fall to the Axis powers. The men understood that they would remain sealed within the facility for about a year, although it could be much longer. Provisions for a seven-year stay had been assembled in the complex. However, the plan was never activated. The Director of Naval Intelligence ordered that the provisions in the complex be distributed and the cave sealed. Rumours of a secret complex, eventually dubbed Stay Behind Cave, circulated for decades in Gibraltar, until discovery of the chambers in 1997 by the Gibraltar Caving Group. The authenticity of the site was confirmed by one of the builders in 1998, and a decade later by one of the physicians. That physician, the last surviving member of the Tracer team, died in 2010.