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In this episode we explore the colossal cave network in Kansas, known as Mammoth Cave. 400 miles of natural tunnels have been discovered and mapped so far, but cavers here believe there is a lot more still to come.
In Croatia, we visit a former Yugoslavian subterranean air base, with hangars, weapons stores and control rooms built under a mountain. The hangar tunnels have aircraft shaped doors in the which would allow Cold War fighter jets to scramble directly onto the runways.
And in Wales, in the UK, one man has made it his life’s mission to rediscover the country’s rich mining history. We head down into a former metal mine that has been reopened and secured, allowing people to enter for the first time in 70 years.
