Underground Worlds

Gibraltar is just 6.8 square kilometres in area, but the size of its role in WWII is immeasurable. Beneath its distinctive landscape lie no less than 55 kilometres of hidden tunnels and caves, a subterranean city carved out by a military workforce. Also, under London, 21st century engineering innovation continues the legacy of Victorian pioneers paving the way for the ultimate super sewer and Europe's largest and most breath-taking natural cave systems - The Postojna Cave in Slovenia.

A look at Sweden's passion for subterranean spaces and how the nation has taken extraordinary precautions to protect itself – by excavating an astounding 65,000 nuclear bunkers. Also, the secret caverns and rock formations in Cheddar Gorge in Somerset, UK and an underground first in Romania.

A look at some of Switzerland 300,000 subterranean bunkers built across history. The extraordinary creation of a local entrepreneur that's shrouded in mystery in Liverpool, England. And, a network of tunnels created by man and today used by adventurous subterranean thrill-seekers deep beneath Slovenia's beautiful landscape.

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

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.

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

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 800 BC.

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.

A look at 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.

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