S1 E4 ∙ Kingdom of Salt ∙

The Wieliczka Salt mine in Poland, some 327 metres deep, where the voids left behind from excavating salt have been transformed into extraordinary spaces, carved by hand.

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All episodes for series 1 of Underground Worlds

S1 E1 · Gibraltar

Incredible feats of human endeavour, revealing what people have built, where, how and why.

S1 E3 · Switzerland

A look at some of Switzerland's 300,000 subterranean bunkers built across history.