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 lies 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.

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

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, an 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 800BC.

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.

This episode explores the now hidden arches and vaults of South Bridge in Edinburgh, Scotland and reveal why this underground world has such a sinister past and how today it has been reimagined. Beneath Naples in Italy, under the city, viewers see how the excavation of the volcanic ‘tuff' to build above ground has left an extraordinary imprint underground. And finally, in Finland's capital Helsinki they've been developing an underground master plan for the entire city by reusing older subterranean spaces and creating new facilities for the population.

Take a look at the Dover War Rooms, Maastricht Underground and Odessa Catacombs all hidden underground.