A Medieval Water Tower in the Heart of Massachusetts.

This historic castle was already in ruins by the 13th century.

This pristine World War II airfield gives a comprehensive look at atomic development and aviation history.

Enormous statue of the Emperor of the Mongols.

A network of ancient tunnels and vaulted chambers with mystical Buddhist reliefs, built for ascetic training.

An immense cave that has served as an outlaw's treasure trove, an underground dance hall, a nuclear fallout shelter, and much more.

In this cobwebbed old operating theater, you can practically hear the screams of the unanesthetized patients.

Built in 1547, these ruins lie on a rocky promontory in Ballintoy jutting into the wild ocean waters below.

A crash site in the heart of polar bear territory has become a beloved local landmark.

An ancient woodland that inspired a young J.R.R. Tolkien.

Natural stone arches form a walkable "cathedral" at low tide, then disappear entirely when the water returns.

A massive underwater sinkhole made famous by Jacques Cousteau.

These Great Lakes sea caves accessible by boat in the summer turn into temples of ice reachable by foot in the winter.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

A 30-acre garden inspired by the principles of modern physics.

The ruins of a 15th-century tower house.

The ruins of a once-grand castle that played host to a tournament with 100,000 attendees.

A maze of underground tunnels beneath one of the most beautiful forts in Serbia.

The largest underground pedestrian tunnel system in the U.S. mostly caters to the people working in the offices above, but provides a great, air-conditioned way to traverse the city.

Off the Florida coast, a whimsical world of marine sculptures are revitalizing coral reefs.

Once one of the most polluted caves in America, now an environmental success story and a museum dedicated to caves.

You don't have to visit the deep blue sea to see these majestic creatures swimming wild and free.

In this breathtaking canyon, crocodiles bask on boulders and glide through the murky waters.

Over 40 species of fern cover the canyon walls of this lush green bowl, cut off from the bustle of the world by a cave entrance.