Man-made mineral caves house massive bat colony.

Disused seaplane hangar gets a second life as the jaw-dropping home of Estonia's museum of maritime history.

The only remaining U-boat that is still floating never actually saw any action in World War II.

The Bauhaus-style former industrial complex is considered the world's most beautiful coal mine.

Hamburgers are intensely proud of their tunnel beneath the river.

An industrial relic is now one of Europe's most staggering art spaces.

A former ironworks that was the first UNESCO industrial site.

This incredible stone temple is a monument to death, victory, and fantasy architecture.

This former steel and coal plant is now a neon-lit industrial fantasia.

This unique Renaissance castle was turned into a home for Nazi occultism.

Soaring over 6,000 feet in the Bavarian Alps, this beer garden was once Hitler's 50th birthday present.

This jaw-dropping 19th-century bridge uses its reflection to form what appears to be a perfect circle.

This Nazi ruin was the site where Hitler was almost blown up by a suitcase bomb.

Warsaw's fantastically preserved gasworks...almost.

The tunnel's elegant proportions keep you snug with no claustrophobia freak-out.

This former quarry is now a picturesque gorge that hides a sunken lake.

This network of conjoined cellars was riddled with traps and supposedly allowed the townspeople to pretend their homes were deserted.

A massive 1830s industrial site once known as Czechia's "Iron Heart" has been transformed into a playground of science, technology, and art.

The deepest sinkhole in Central Europe comes with a grisly tale and an unusual nickname: “The Stepmother Abyss.”

A series of caves under Castle Hill that once held the prisoner Vlad Tepes, better known as Count Dracula.

A marvel of technology a century ago, this disused power station now offers a lovely glimpse into the industrial design of the past.

Austria's very first and only nuclear power station was completed but never used.

Vienna's underworld of tunnels and subterranean rivers made an iconic appearance in a 1949 Orson Welles film.

A nuclear reactor created for Eisenhower's "Atoms for Peace" campaign is now used for research at a Dutch university.