Collapsing castle on the Hudson river was once the storage site of military surplus business.

The largest self-built castle in the U.S., six decades in the making.

A museum dedicated to the history of a terrifying secret police is now held in their former headquarters.

Rebuilt remains of a temple to Roman god Mithras.

If drivers cross the rumble strip on this stretch of Route 66 at just the right speed it plays "America the Beautiful."

The quaint little store that is said to have inspired a famous Dickens novel was only given its name after the book was released.

Frank Lloyd Wright's most iconic home dangles over a Pennsylvania waterfall.

A modernist landscape of water and sci-fi concrete.

This naturally-sourced hot spring is now home to a clothing-optional resort.

Mural made of 100,000 pieces of hand blown glass, and until recently, the largest glass mural in the US.

The world's longest known cave system.

This classic restaurant features a gorgeous mirror-clad room.

One person's lost luggage is another person's treasure.

This volcanic cave lake is essentially hard proof that Iceland is the most magical place on Earth.

Become a prohibition rebel.

This fantastical waterfall is hidden in a cave located in a cracked cliff.

A 1,215-foot tunnel transports pedestrians beneath the River Thames.

The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

A lush green oasis in the middle of Chicago.

The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

A house carved into a rock in the 1940s.

The magical, mossy forest that inspired Princess Mononoke.

Where manatees are the snowbird vacationers in these balmy Florida springs.

If Hello Kitty were Mickey Mouse, this would be Disneyland.