This abandoned Brutalist treehouse is a hidden architectural relic.

This Italian foot path winds through the mountains via 52 individual tunnels.

This private skyway is filled with art and was expanded explicitly so that Hitler could appreciate a river.

This real life asylum of horrors is now an abandoned ruin that hides the unbreakable code to one patient's insanity.

This shining multicolored metro station deep under Naples is said to be the most beautiful in Europe.

Dug as a secret royal escape route, the tunnel became a wartime bomb shelter and dumping ground for vintage cars.

Filled with goldfish swimming in a shallow pool of sea water, this ancient crypt is more wishing well than burial site.

Abandoned medieval hill town.

Skulls from the plague of 1656 and World War II bombings are united by a cult dedicated to caring for the ancient dead.

This beautiful park on the outskirts of Rome protects the ruins of two colossal ancient aqueducts.

A labyrinth of ancient tunnels hidden below the city holds the ruins of 2,500 years of history.

An Italian hillside ghost town.

Hidden in an Italian forest is a completely homemade amusement park that has taken a lifetime to create.

Sitting on an Italian hilltop is this surreal fortress built by the mad inventor of electrohomeopathy.

Geothermal waterfalls in Tuscany.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

A Faraday Cage protects cars from a nearby high-power transmitter.

Silent memorial, using bone conduction to take people back to the Dresden Bombing of 1945.

The 1936 Summer Olympics site is a crumbling remnant of sportsmanship and a history its host country would like to forget.

A decomissioned facility where the German "Transrapid" maglev trains were once tested.

Hidden in the forest are the remains of a former World War II chemical plant.

In a divided Germany, Potsdam's "central" train station was relocated to the city's southern suburbs to avoid West Berlin.

Tallest - but not most powerful - wind turbine in the world.

Power line with pylons on huge concrete bases to prevent terrorist attacks.