00:00Man-made underworlds have come a long way since World War II.
00:06Today, there are over 10,000 underground military installations around the world.
00:12Some, like the Cheyenne Mountain Complex in Colorado, are cutting-edge military cave structures.
00:19Built at the height of the Cold War as a nerve center for nuclear missile defense,
00:24it remains one of the US's most secure surveillance bases.
00:30Located under almost 2,000 feet of granite, with 25-ton blast doors,
00:36this huge complex can resist almost any type of attack.
00:43It's a far cry from Gibraltar's wartime tunnels, but it shows just how vital caves remain for our protection.
00:51We've lived underground for hundreds and hundreds of thousands of years,
00:55and the fact that even though we have 21st century technologies and surveillance systems,
01:00we go back to the caves to try and protect those is a really interesting image.
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