Underground Worlds - Season 2

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.