Underground Worlds - Episode Guide

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.

Exploring the forgotten underworld hiding beneath the streets of Cincinnati, USA. Late 19th Century Cincinnati was a hub of German immigrants who introduced lager to the United States. To make their beer they needed cool temperatures, so they headed underground.

Also, a state-of-the-art Cold War fortress on the shores of Denmark. These dark tunnels were once the first line of defence for NATO in their battle against Russia and her allies. The clifftop guns above the caverns packed enough firepower to hold the Russian Baltic fleet while reinforcements arrived.

And, there's a deep dive into the tunnels spanning the Mersey River in Liverpool. Driven by the rise of the automobile and increasingly clogged ferry routes, the Queensway Tunnel was both a feat of engineering and a world first when it opened in 1934.

Inside the Grand Canyon Caverns, North America's largest dry cave system, the eastern European cave city of Vardzia in Georgia, plus, the hidden UK Cold War bunkers which played a pivotal role in monitoring for incoming nuclear bombers. Peek inside a bunker which has never been seen on TV before, and another that has been faithfully restored to how it would have been when the Cold War tensions were at their highest.

The forgotten underworld of beer brewing tunnels hiding beneath the streets of Cincinnati.

Plus, a state-of-the-art Cold War fortress on the shores of Denmark designed to keep the Russian Baltic fleet at bay. And the historic automobile tunnels guiding cars under the Mersey River in Liverpool since 1934.

A top secret, Cold War US government bunker is hidden beneath the Greenbrier Hotel in Virginia; in a former slate mine in Wales, the shafts and drifts have been brought back to life. In central Israel a vast 2000-year-old cave network has stayed unchanged below ground, while towns and cities have grown and fallen above.

Exploring the vast 400-mile Mammoth Cave in Kentucky, USA. Plus, a newly constructed opening into a metal mine in Wales - untouched for over 70 years.

Exploring hidden defences that have kept Cork Harbour, in Ireland, safe for over 400 years. Plus, a former submarine base in the fjords of northern Norway.

Exploring long narrow tunnels beneath Exeter that were built in medieval times to protect the city. Plus, a marble quarry under the pine forests of Vermont.

A mile underground in a former gold mine in South Dakota, now lies a world-leading scientific laboratory investigating neutrinos. A huge network of limestone tunnels under a quiet London suburb played a crucial role the subterfuge that protected the D-Day landings. And the pitch blackness of a former water cistern in Denmark with new and unexpected residents.

Exploring old tunnels in an abandoned tin mine beneath the moors of Cornwall. Plus, former Atlas missile siloes in Kansas and tunnels hiding under Valetta.

Exploring caves beneath Orvieto in Italy, originally built by the Etruscans long before the Roman Empire. Plus, a former limestone mine under Kansas City.