World's first "penitentiary," meant to be humane, drove men insane.

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

This historical Netherlands fortress still bears the marks of its single siege.

A monument hidden in the woods built in 1938 as a getaway for a Dutch museum curator.

The ruins of this Dutch castle sit atop a labyrinth of once-secret tunnels and passageways, dating back over 900 years.

Hidden within a Netherlands crater is a stone bed that lets those who lie in it see the sky as a curved surface.

This beautifully designed star-shaped fort was the premier defense system of the late 1500s.

Museum preserving torture methods from the past.

This Polish cave has housed a dragon, a brothel, and an enduring legend.

A walk through the historic city center, under the streets of Seattle.

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

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

A roaring, awe-inspiring double cascade of icy water flows through woodland Oregon like something out of a Tolkien novel.

The real life Hobbiton, this sheep farm has been forever transformed since being built for The Hobbit film series.

The ghostly remains of an accidental sea.

The inspiration for H.P. Lovecraft's horror stories and Batman's Arkham Asylum is now a condominium complex.

A massive underwater sinkhole made famous by Jacques Cousteau.

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

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

Medicinal spa created with the discharge from a geothermal energy plant.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.