We travel to new depths to uncover the wonder of the Wieliczka Salt mine in Southern Poland, some 327 metres, nine stories deep, the empty voids left behind from excavating salt have been transformed into extraordinary spaces, carved by hand, under the UK's capital, London, we reveal the subterranean structures constructed to protect its citizens during World War Two and in Slovenia a castle built into the face of a mountain hides a magical underground world built both by hand and by nature

An underground city beneath a forest in Northern France built to house 3,000 people, an ice cave under Iceland's biggest glacier, and a subterranean power station in London.