Season 4

A settlement in Argentina destroyed by floods, a town in the US that has been erased, and a Soviet dream-town that turned into a nuclear nightmare.

A seemingly endless concrete line stretching through the French countryside, which sits as a monument to a discarded engineering advancement.

A little-known dark past explains the vast monolithic structures that dominate the small British island of Guernsey.

Projects include a maze of tunnels filled with haunting remains beneath the streets of Paris and a strange formation of domed structures hovering over the waters off Florida.

A town in West Virginia that fell victim to the pace of change when steam trains became obsolete and an underground complex in Italy that had been abandoned for nearly 2,000 years.

Features a ruined compound in Colombia which was once home to Pablo Escobar and alien-looking structures with no apparent purpose, isolated in an empty English marshland in Denge, near Dungeness in Kent. But how and why were they ever built at all, and why are they now left abandoned?

A formerly luxurious and strategic mountain-top villa on the island of Rhodes in the Mediterranean that's been ravaged by acts of revenge by locals since 1955.

A strange-looking deserted town with an almost supernatural atmosphere on a remote Norwegian archipelago in the Arctic Ocean was in fact an ambitious Soviet mining outpost with a tragic history. Also, a collection of imposing buildings on the southern tip of Australia, the remains of curious-looking basins emerging from the waves in the shadow of the Golden Gate Bridge and the ruins of a monumental construction project in southern Germany that was a ruthless regime's deadly prototype.

Still standing today, a fierce-looking prison and an innocuous looking office building in Berlin were actually the headquarters of a feared and ruthless organisation - the Stasi. Also, a covert Communist base hid Cold War submarines for Tito, the former leader of Yugoslavia on the island of Vis. A seemingly innocuous sports centre hides a sinister underground bunker. And the former HQ of the Soviet Army in East Germany was a forbidden Soviet city within a city.

A sprawling Wild West ghost town sits peaceful and abandoned in Montana, USA, but is still haunted by tales of murder, corruption and buried treasure. Also, a cold, dark, underground mountain network that cost the lives of thousands... the crumbling ruins of an English industry that turned poison into profit... a peaceful village that was shattered by the terrors of war.

A vast snowcovered monster in the icy wilderness of southern Alaska, a ruined super structure that overshadows everything around it in and a landscape of confusing and surprising structures that once represented a bold statement of Spain's plans for the future. Plus, a long, winding formation that was once supposed to help defend Hitler's Third Reich. And in Budapest, Hungary, the stripped skeleton of an industrial relic lies rusting away.

A pioneering complex marked by both glory and tragedy that's soon to be lost forever, and towering remains from an industrial age.