Season 1

2019-09-25 · 45m

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.

2019-10-02 · 45m

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.

2019-10-09 · 45m

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.

2019-10-16 · 45m

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.

2019-10-23 · 45m

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.

2019-10-30 · 45m

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.

2019-11-06 · 45m

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.

2019-11-13 · 45m

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.

2019-11-20 · 45m

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.

2019-11-27 · 45m

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.