In this episode, Rob Bell discovers the gruesome secrets hidden in an ancient Scottish cave, a 1950s engineering wonder buried inside a mountain, some prehistoric animal bones that are exciting scientists researching the Ice Age, and a vast man-made cavern that helped Britain win World War II.

This week, Rob reveals a mysterious side to Englandʼs capital that very few people ever get to see. Beneath the bustle and noise of the sprawling, teeming, traffic-congested city streets lies a world of dark forgotten places. Prising open unassuming entrances and overlooked doorways, he finds a labyrinth of tunnels, vaults and even secret places that the public were not supposed to know even existed.

In this episode, Rob explores two natural wonders in the Yorkshire Dales - a mile of caves and one of Britain's deepest potholes. In Stockport, Rob learns how its unique geology made it the perfect spot to build the country's largest air-raid shelter and he discovers the incredible depths that are being plumbed to retrieve agricultural fertiliser.

Rob visits Europe's largest cave system underneath Brecon Beacon national park, the dungeons of Chirk castle and explores the hidden remains of Ruthin castle. He also abseils down a cliff in the Gower Peninsula to a cave reputed to contain the bones of a prehistoric prostitute.

Under the city of Nottingham Rob explores the largest system of man-made caves in the country, before uncovering a bloody secret below a humble back garden. In Norfolk he learns about Britain's first industry that was established 5,000 years ago and he also explores an 18th Century den of debauchery below ground level in Buckinghamshire.

Rob explores a mystical cave system that altered Victorian Britain's view of human history before entering a once top-secret bunker that was constructed to be Britain's nerve centre if the bomb had been dropped during the Cold War. He also follows an underground seam of tin in a mine that stretches out several miles under the Atlantic Ocean.