Season 2

Exploring the extensive air raid tunnels hidden beneath Ramsgate. Also, how the former thriving mining town of Butte, Montana repurposed the 10,000 miles of tunnels.

Exploring inside the Grand Canyon Caverns - North America's largest dry cave system. Plus, Cold War radar bunkers around the UK.

Exploring the forgotten underworld hiding beneath the streets of Cincinati, USA. Plus, a state-of-the-art Cold War fortress on the shores of Denmark.

Exploring the secret, Cold War US government bunker hidden under Greenbrier Hotel, West Virginia; Plus, delve into the depths of a former slate mine in Wales, UK.

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.

Exploring a former gold mine turned scientific laboratory in South Dakota. Plus, limestone tunnels in London and a former water cistern in Denmark.

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.