The Nekoma Pyramid, North Dakota, USA
Costing the US Military over $6 billion, the Nekoma Pyramid was developed in the 1960s to protect the US from incoming intercontinental ballistic missiles fired by the Russians. The massive complex had missile silos, a radar system, and launching silos that could be used for surface-to-air missiles. The pyramid was also equipped with a “backscatter radar” site that could follow Russian missiles and shoot them down before they reached the US.
Source: Reddit
Due to the complex’s expense, questions about its effectivity, and the danger of exploding a ballistic missile over Canada, the House Appropriations Committee voted to shut the program down. After less than a year of operation and less than 24 hours of full operational capacity, the Nekoma Pyramid was officially closed in February 1976.