Hot springs monkey spa in the middle of Hell's Valley.

The old Niles Canyon Aqueduct in the San Francisco Bay Area is a bemusing and secret trail through nature for wayward travelers.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

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

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

Mother Nature outdoes herself with this stunningly beautiful set of caves carved into marble.

A 16th-century horror show built in a lovely Italian garden.

The history of occult science is turned into a creepshow at this lurid Prague tourist attraction.

A swing to dangle you off the very edge.

Tucked in the back of this amazing antique store is an important piece of medical history.

Dramatic river bend surrounds a natural red-rocked pedestal.

The legendary building that needed ghostly approval before being built.

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

A massive underwater sinkhole made famous by Jacques Cousteau.

A toxic ghost town sitting atop a massive coal fire.

A bizarre house filled with an astounding array of collections.

An eerie, derelict amusement park in Berlin.

A crumbling hospital from the 1850s on Roosevelt Island.

This beautiful building in the heart of Manhattan's Financial District was empty for decades.

An ancient capital city that dates back 3,000 years old and harbors many surprises.

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

The fairy tale castle of the "fairy-tale king."

A fiery crater has been burning in the Karakum Desert since 1971.

The rocks in this arid basin leave trails of movement behind them.