20. Cheyenne Mountain Complex, United States
The Cheyenne Mountain Complex is a military installation and nuclear bunker in Colorado, located at the Cheyenne Mountain Air Force Station. It houses or has at one time housed the United States Space Command, the Aerospace Defense Command, the Air Force Command and the Federal Emergency Management System. The complex is a fascinating web of buildings built 610 meters under granite that is protected from natural disasters. It boasts flexible piping systems and has its own power plant and water supply. Oh, and it can withstand a 30 megaton nuclear explosion, bombs and radiological contaminants. Good luck finding a way in.
19. HavenCo, Sealand