This series of Scottish caves has no clear purpose other than baffling historians.

This long, dark tunnel is open to hikers, but those without lights are known to get confused even on the straight path.

Perched on a spindly rock spire in a Greek archipelago, this lighthouse would make a perfect wizard's tower.

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

Where America's first doomsday cult awaited the end of the world.

A book bound in the skin of an infamous criminal, along with countless other specimens.

This former Masonic hall has been turned into an art project that uses even more metaphysical iconography.

A vast city of the dead, where the deceased outnumber the living by more than a million.

Small, privately owned island divided between Michigan and Ohio with abandoned vacation homes and a derelict lighthouse.

360-degree virtual reality chamber physically immerses scientists into their data.

The curious underground history of keeping Paris clean.

An illegally built ramshackle mansion tucked in the middle of Brooklyn is now being turned into condominiums.

The second-longest continuous wall on the planet protects a hidden secret in the deserts of India.

Protecting the future of some of the rarest and most misunderstood mammals of North America.

"The most colorful grottoes of the world.

Who needs fancy modern water slides when this giant North Carolina rock works just as well?

The stately remains of Llywelyn the Great's mighty fortress.

Yugoslavia's biggest underground airport lies in ruins.

Discover for yourself what Herb Caen referred to as "the best kept secret in the Bay Area."

An ancient stone fortress considered by many to be the eighth wonder of the world.

The remains of a ruined mansion memorialize the 1906 earthquake.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

This little-known collection holds everything from "Dracula" notes, to the manuscript of "Ulysses," to the forged works of Shakespeare.

Abandoned Nazi train station turned underground astroparticle laboratory.