Season 1

01 Gibraltar Episode 125th September 2019 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. 02 Sweden Episode 22nd October 2019 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. 03 Switzerland Episode 39th October 2019 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. 04 Kingdom of Salt Episode 416th October 2019 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. 05 Bouzincourt Episode 523rd October 2019 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. 06 City of Naours Episode 630th October 2019 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. 07 Jersey War Tunnels Episode 76th November 2019 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. 08 The Maginot Line Episode 813th November 2019 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. 09 Edinburgh Vaults Episode 920th November 2019 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. 10 Dover War Rooms Episode 1027th November 2019 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.