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Gibraltar

Episode number: 1

Overview: This edition explores the hidden network of tunnels and caves beneath Gibraltar that were instrumental to the Allies success during WWII. Plus, the ultimate super-sewer beneath London and a breathtaking cave system in Postojna, Slovenia.

Sweden

Episode number: 2

Overview: Explores how Sweden has gone to extraordinary lengths to protect itself - by excavating 65,000 nuclear bunkers. Plus, Somerset's Cheddar Gorge and an underground first in Romania.

Switzerland

Episode number: 3

Overview: A look at some of Switzerland's 300,000 subterranean bunkers, the creation of a local entrepreneur shrouded in mystery in England and a network of tunnels deep beneath Slovenia.

Kingdom of Salt

Episode number: 4

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

Bouzincourt

Episode number: 5

Overview: A labyrinth of tunnels underneath a tiny French village which contains mysteries from several historical eras, and a Welsh slate mine that has been given a new lease of life.

City of Naours

Episode number: 6

Overview: 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.

Jersey War Tunnels

Episode number: 7

Overview: The underground bases built by the German army during the occupation of Jersey, and a group of more than 20 underground cities in Turkey, one of which dates back to 800BC.

The Maginot Line

Episode number: 8

Overview: The secrets of the largest subterranean fortress along France's border with Germany, and a look at how a unique precious mineral was discovered in Derbyshire.

Edinburgh Vaults

Episode number: 9

Overview: The sinister history of Edinburgh's vaults and the development of Helsinki's subterranean spaces. Plus, the impact of the building of Naples on the volcanic ground below.

Dover War Rooms

Episode number: 10

Overview: This edition explores the military headquarters built beneath Dover Castle during the Second World War, and Odessa's catacombs, possibly the largest tunnel network in the world.