This hill and tower are steeped in legend, from King Arthur to the Holy Grail.

One of nature's most well-scheduled phenomenons resides in Wyoming's Yellowstone National Park.

An architect's childhood fantasy treehouse made real.

The only retired U.S. nuclear missile silo that is open to the public.

One of the smallest towns in the United States.

A popular rock that acted as a waymarker for American pioneers.

The first declared National Monument in the United States.

A magical sinkhole on the Yucatán Peninsula is adorned with mini waterfalls and hanging vines.

No telescopes, no power, no gift shop: The Observatory of Chichen Itza, one of the world's oldest observatories, takes astronomy back to basics.

A notoriously corrupt Chief of Police's deteriorating Greek-themed playhouse.

Every equinox this Maya pyramid puts on a spooky ancient light show.

Clap your hands at the base of the pyramid, and the song of a sacred Mayan bird will echo through the air.

An underground nightclub helped spark a "Singing Revolution."

A Nesting Doll of Churches.

This seaside Italian village doesn't just look Escher-esque, it actually inspired the mind-bending artist.

The little-known second castle that William the Conqueror built in York.

Perched above the bustling town of Kerak is a sprawling, ruinous castle that has witnessed nearly a millennia of Levantine history.

This serene, turquoise lake in an extraordinary nature reserve showcases a rare natural phenomenon.

This breathtaking monastery was established by Saint Benedict himself 1,500 years ago.

This medieval settlement was abandoned after a series of landslides. What remained was largely destroyed during a mock battle for a WWII film reel.

A ruined castle stands atop a hill surrounded by dragon lore.

This former abandoned hospital, a complex of stunning 19th century buildings, once housed 2,000 psychiatric patients.

This monastery built into a cliff hides a perfectly preserved hermit in a glass coffin.

An abandoned lighthouse awaits its destiny: being buried under shifting sand.