Project Iceworm (with public cover name Camp Century for the top secret project) was one of the most Bond villain undertakings of the Cold War. Between 1960 and 1965, the U.S. Army built a city in tunnels beneath the Greenland ice cap at an altitude of 2000 metres above sea level. Power was supplied by a mobile nuclear reactor. The Danish government, which has sovereignty over Greenland, was told only that the facility was to study construction techniques in arctic environments and a base for scientific research on the ice cap.
In reality, the goal was to study feasibility of basing 600 nuclear-tipped medium range ballistic missiles on mobile launchers which would move to random locations in an enormous web of tunnels beneath the ice. Wikipedia describes the concept as follows:
According to the documents published by Denmark in 1997, the U.S. Army’s “Iceworm” missile network was outlined in a 1960 Army report titled "Strategic Value of the Greenland Icecap ". If fully implemented, the project would cover an area of 52,000 square miles (130,000 km²), roughly three times the size of Denmark. The launch complex floors would be 28 feet (8.5 m) below the surface, with the missile launchers even deeper. Clusters of missile launch centers would be spaced 4 miles (6.4 km) apart. New tunnels were to be dug every year, so that after five years there would be thousands of firing positions, among which the several hundred missiles could be rotated. The US Army intended to deploy a shortened, two-stage version of the U.S. Air Force’s Minuteman missile, a variant the Army proposed calling the Iceman .
In fact, construction and operation of Camp Century revealed that the ice cap was too unstable to support long-term construction projects, with compression and migration of the ice constantly deforming the tunnels. The nuclear reactor was removed in 1963 and the facility decommissioned in 1967.
Plans for the missile base remained secret for decades until they were discovered by an investigation by the Danish Foreign Policy Institute into the history of basing nuclear weapons in Greenland.