The 'Most Secure Place on Earth' marks 60-years: NORAD base buried deep inside Cheyenne Mountain opens its 23-ton blast doors as the Cold War relic reinvents itself for the 21st century

Behind 23 ton blast doors (main) lies what it known as the most secure place on earth - a warren of tunnels blasted out of Colorado granite in the 1960s teeming with missile monitoring equipment. NORAD called off its 'nuclear watch' in 1992 after the Soviet Union disintegrated, but the site is still monitoring threats to the US around the clock - including, despite the recent detente, Kim Jong-Un's North Korea. Also pictured is the entrance to the main tunnel (top right) and an officer demonstrating monitoring equipment in the main command room.