In the late 1950s, the United States Army (USAR) started carving ice tunnels in the Greenland ice shelf. The tunnels formed Camp Century, secretly known as Project Iceworm. Publicly, Camp Century was a research and development station. Ice cores sampled during this time are the foundation for scientists climate change assertion. The first functional nuclear power station, as part of the Army’s Nuclear Power Program, went CRITICAL in Camp Century – the reactor was called PM-2A and built by ALCO. Project Iceworm’s mission was to determine the feasibility for the Greenland ice shelf to support nuclear missile launches.
A couple of decades after Camp Century was abandoned, a 1962 memo was found, detailing Project Iceworm’s proposal. The USAR really wanted to determine the feasibility for the Greenland ice shelf to support 600 nuclear missiles, targeted at the Soviet Union. The missiles would be moved around, under the ice sheet, by rail. The warheads would be safely concealed under 28 feet of ice, with a surface area equal to the state of Alabama.
In 2013, the United States Air Force Nuclear Weapons Center was planning to award several study contracts, worth up to $3 million each. The goal of the study contract was to figure out how to extend the operational life of the Minuteman III missiles.
There were two possible outcomes that the Air Force was looking to pursue, (1) maintain the current Minuteman III missiles until around 2030 or, (2) replace the current setup with a new ICBM that would be hidden within a trackless, underground subway system. The cost of developing a complex underground system would be a considerable, with figures between $52 billion to $210 billion.
You can read the 2013 United States Air Force Counterproliferation Research & Education article at https://media.defense.gov/2019/Jul/24/2002161886/-1/-1/0/CPC%20OUTREACH%201049.PDF. I did not want to post the article here because of possible copyright issues.