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Gibraltar's geographical position has meant it has long been a fortress. It is honeycombed with 34 miles (55km) of tunnels constructed over 200 years.

The Travel Show's Christa Larwood takes a tour and visits the more recently discovered "stay behind" cave created during World War Two.

It was planned that if the Gibraltar fell into enemy hands six people would go into hiding for up to a year to spy on any occupation forces and report back to Britain by radio.