English: Operation Tracer Cave was a highly classified British military operation based in Gibraltar during the course of WW II.

In 1941, Royal Naval Intelligence decided to establish a covert observation post at Gibraltar that would remain operational even if Gibraltar was captured by the enemy in the event of the Germans activating Operation Felix. Operation Felix was the codename for the plans by the German High Command to capture Gibraltar by an attack from neighboring Spain. It was intended that, from the secret observation post, the movements of enemy vessels in the Straits and Bay of Gibraltar would be reported back to the UK. The decision was made to construct the facility using the existing tunnel system for Lord Airey's Shelter an underground shelter near Lord Airey's Battery. (For more on Lord Airey’s Battery see the set on this Battery within this Photostream).

The cave complex comprised living quarters, transmission room, sanitation and dual observation posts. The complex included an observation slit overlooking the Bay of Gibraltar (2 cm wide) and a larger opening over the Mediterranean Sea. Royal Naval Intelligence selected six men for the operation and it was understood that they would remain sealed within the cave complex for about a year or longer with provisions for a seven-year stay assembled in the complex.

Operation Tracer was never activated once the risk of Gibraltar falling into enemy hands became increasingly low.

More information on Operation Tracer can be found in en.wikipedia.org/wiki/Operation_Tracer