'Tracer' (i) was a British unrealised plan to seal six men, with food, water and other essential supplies for seven years, inside the Rock of Gibraltar should this key base appear likely to fall to the Germans (June 1940/August 1943).

After the fall of France in June 1940, British intelligence came to believe that the German forces would attempt at some time to advance through Spain to take Gibraltar from the landward side. It was decided to make caverns in the rock to hide the men, who would be able to observe subsequent German movements by means of two slits, each measuring 12 by 6 in (305 by 152 mm), looking to the east and west, and then transmit, with the aid of an 18-ft (5.5-m) antenna which could be lowered out of either of the observation slits, details of German targets for attack by British warplanes.

The caverns were to be 45 ft (13.7 m) long by 16 ft (4.9 m) wide by 8 ft (2.4 m) high.

The planning for and possible implementation of 'Tracer' (i) were discussed only in the home of Rear Admiral Sir John Godfrey, the Director of Naval Intelligence, and specialist advice was solicited from George Murray Levick, who had been on Captain Robert Scott’s ill-fortuned expedition to the Antarctic in 1910/13 and was now called up, at the age of 64, with the rank of surgeon commander. Levick advised on the selection of suitable volunteers, survival techniques, diet, exercise, recreation, alcohol, tobacco, clothing, ventilation, sanitation, and the disposal of any dead by means of embalming and cementing-up. Levick also suggested a rehearsal, probably in Scotland, to test the temperamental suitability of the personnel.

Finally, volunteers were taken to the naval school at Shotley, Suffolk, for training, before being sent to Gibraltar with 'proper jobs' as cover for the real purpose of their mission. MI6 provided training in radio techniques and also supplied the radio equipment and a bicycle-operated generator.

The scheme was deemed to be so full of potential that similar operations were planned for Colombo and Trincomalee in Ceylon, Malta in the Mediterranean and Aden at the lower end of the Red Sea.

In August 1943 the decline of any real German threat to Gibraltar led to the termination of 'Tracer' (i), the personnel being stood down and the caves being sealed.