Operation Tracer was a top @-@ secret British stay @-@ behind spying mission that was only to be implemented if Gibraltar was captured by the Axis Powers . Six men were to be sealed in a cave and left with only enough supplies for a year . The volunteers — two doctors , three signalmen and their leader — would run an observation post with one 12 @-@ inch ( 300 mm ) by 6 @-@ inch ( 150 mm ) slit looking over the harbour and a concealed outdoor terrace over the Mediterranean . The team would then wire back all shipping movements to the British Admiralty .