The Tunnels of Gibraltar were a Second World War espionage base, hidden in the Rock of Gibraltar, which lay undiscovered for decades. At Atlas-1 Test Site, are the remains of an ingenious US Airforce project that continues to keep people safe.

Exploring a series of vaults, hidden from view in Edinburgh, Scotland that tell a murky story of opportunity, crime and destitution.

Exploring the remains of an ingenious sky-high solution to a ground-based problem, Chiatura, once know as the Venice of the air

Exploring Agdam, a city in Azerbaijan raised to the ground in a conflict decided by a new weapon of war.

Exploring an industrial ruin in the Florida Everglades that once competed in the greatest race humankind has ever witnessed.

Exploring traces of a mass deception hidden at Ponte Dirillo in Italy, as well as a once lavish theatre in Ohio and an ancient city with a poisonous secret.

Explores a dense forest near the Czech capital of Prague that hides Houska Castle, an eerie keep that has been subject to long-standing rumours that it possesses supernatural powers.

Exploring Serbia, Fort Pike in Louisiana, and a mountainside refuge in Georgia that was revealed to the world by an act of God.

Exploring a grand structure in Hull that was reduced to rubble by Hitler's planes, Casa Albero in Italy, USS Hornet in California and the Sofia tunnels.

Exploring a decrepit Liberian landmark with American origins, a desolate town in Belgium and a pioneering scientific structure in Cleveland.

Exploring the Dew Drop Inn in New Orleans that dared to take on the authorities and paid the price and Belgica Fort, built to defend an unlikely treasure.