“During the Cold War, Norway built a secret naval base, Olavsvern, that was carved into the mountain just outside the city of Tromsø, in the Arctic Circle. The base—comprised of a submarine hangar made of rock, direct sea access, nearly 150,000 sq ft of buildings and almost double that in bombproof interior space—took 30 years and cost NATO around $500 million to build to fight the threat of the Soviet Union.”
Source: How Norway Lost Control of Its $500 Million Arctic Sub Base to the Russians – Defense One
