Given that Gibraltar was a small town with only a few defences protecting it , the solution was to build a massive series of tunnels and chambers inside the natural protection of the Rock of Gibraltar . This " town " inside the Rock contained its own power station , water supply , and hospital . Some soldiers posted here would not see the light of day for months on end . Two Canadian engineer companies , the only soldiers with diamond @-@ tipped drills and 5 British engineer companies , added some 30 miles ( 48 km ) of such tunnels , a feat thought impossible at the time . That was enough to hold all 30 @,@ 000 troops on the rock . Today , the rock has more underground tunnels than roads .