One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

The underground sanctuary of a mysterious ancient cult.

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

Ruins of a German Castle on top of a towering stone pillar.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

This lab's maddening silence is good for business but bad for sanity.

Northern Ireland's Brobdingnagian stepping stones.

A spectacular cave and a beguiling lake.

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

An isolated dive site teeming with cnidarians.

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

A seemingly endless landscape of pure salt stretches far across Bolivia.

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

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

A massive underwater sinkhole made famous by Jacques Cousteau.

The sign at the garden gate reads: "These Plants Can Kill."

Rippling sand dunes frozen in the Arizona rock.

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

This stunning mass of ice spans 121 square miles and is still growing.

One of the most extreme deserts in the world, these snow-free valleys harbor life, despite the hostile environment.

Sacred Buddhist home of some spiritual monkeys.

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.

A battle between nature and architecture in the Cambodian jungle.