In 1940 an urgent request was issued by the British Government for hardrock miners to work on the rock of Gibraltar. Canada responded by sending Noranda Mine workers who became part of the Royal Canadian Engineers and began one of the most impressive tunelling work during WW2. The task of the workers was to carve out a cavern in the rock (the whole of Gibralter is less than 3 miles wide) In fact,, a total of 48 kilometers of tunnels were escavated.
According to Wikipedia, Inside the Rock of Gibraltar itself, miles of tunnels were excavated from the limestone. Masses of rock were blasted out to build an “underground city”. In huge man-made caverns, barracks, offices, and a fully equipped hospital were constructed, complete with an operating theatre and X-ray equipment.