Unusually, this ancient Iron Age broch has never been excavated.

The last occupied troglodyte dwellings in England weren't abandoned until the 1960s.

The oldest surviving presidential residence in the United States.

Explore the dark underground corridors of this graffiti-covered former military fortification.

On the edge of the Valles Caldera National Preserve, these rock formations create an eerie landscape.

Rare recess caves fill this unbelievably beautiful Ohio park.

What is it about mushrooms that makes people wish they lived inside of them?

This amazing 15th-century mural remained hidden in plain sight for centuries.

A perilous medieval road leading right into the sea.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

This fast-food outlet is the only space and UFO-themed McDonalds in the world.

This atmospheric spot in Pärnu's historic old town is located in a 17th-century almshouse.

Hidden behind a gate in Greenwich Village is a little farmhouse that once served as the writing studio of a bestselling author.

A harbor town pays tribute to a heroic wartime carrier pigeon.

One of the oldest seagoing boats in the world can be found at the Dover Museum.

An Irish church where an 800-year-old mummy is reaching out of his coffin as if to shake hands.

This abandoned building made of petrified wood was once a hotspot for buying moonshine.

This small shop walked so America could run on Dunkin'.

An underwater city for the dead.

The most visited, and possibly most controversial, museum in Germany.

This tower once belonged to a castle fit for a knight.

A 140-year-old castle with underground caves hidden in San Francisco.

The ruins of an ancient island city built atop a coral reef in the South Pacific are spectacular and strange enough to have inspired H.P. Lovecraft.

The ruins of a medieval castle that once belonged to one of the most powerful families in the Netherlands.