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

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

This Nazi-built amphitheater was constructed on a sacred mountain site used by various German cults.

The birthplace of an eccentric alchemist and possible inspiration for the fictional reanimator of monsters.

This Luxembourg cave is covered in cryptic looking marks left by mining activity.

Underneath a chalk hill in France lies what Hitler hoped would be the underground launching base for a devastating new weapon.

The vast chamber inside this abandoned cooling tower looks like something out of a sci-fi dystopia.

An abandoned saucer-shaped monument in the Bulgarian mountains.

This brutalist monument is inscribed with 9921 names of Yugoslav partisans who perished during WWII.

Yugoslavia's biggest underground airport lies in ruins.

This huge metal monument is slowly being stripped to its concrete skeleton.

This spaceship-like Serbian monument reaches for the stars in celebration of the fallen fighters of fascism.

These warring psychedelic crags commemorate an unsuccessful military operation.

This massive concrete sculpture is said to be the heaviest communist monument on Earth.

Space-age monument to an early 20th-century uprising.

This massive stone and steel monument remembers a century-old battle that cost thousands of lives and nearly destroyed the city.

This stunning, out-of-the-way Croatian monument commemorates a community's uprising against occupying fascist forces.