A church of bones, decorated with 40,000 human skeletons.

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

In a long-dry lake bed, these groups of massive stone pillars bear a striking resemblance to clumps of everyone's favorite fungi.

Ancient snack bar of the Roman Empire re-opened.

Pompeii's less famous neighbor is still revealing her secrets.

This sword isn't waiting for a king to pull it out. It was placed there by a saint.

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

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

An underground paleo-Christian burial site composed of the remains of several cemeteries and Basilicas.

A sulfuric hellscape in southern Italy.

This medieval Italian town is a sheer (and beautiful) cliffside defense.

This domed temple was built in a hidden cave mouth as a refuge for sinners seeking absolution.

The Roman Emperor's lost summer home and grotto full of enormous sculptures sit along the Italian seashore.

What looks like a link in Gondor's early warning system is actually a centuries-old Italian fortress.

Romans built the world's tallest artificial waterfall in 271 BC.

Ruins of a 10th century church tucked away deep in a botanical garden.

An elegant chandelier hangs over this ancient sewer chamber, now open for concerts and public tours.

Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

An illuminated manuscript comprising the life's work of a lone monk, inexplicably decorated with a portrait of Satan himself.

Housing the groundbreaking maps and globes of Gerard Mercator.

A Museum of Psychiatry in a still-operating mental institution.

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

These medieval skyscrapers dominate the skyline of this small Tuscan town.

On rare days, this abandoned castle's gatekeepers open its doors, revealing the eye-popping splendor hidden inside.