A 300-yard long natural tunnel runs through a ridge deep in the Appalachian Mountains.

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

The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.

Explore the largest pure gypsum deposit in the world, and go dune sledding while you're at it.

A roaring, awe-inspiring double cascade of icy water flows through woodland Oregon like something out of a Tolkien novel.

The first declared National Monument in the United States.

A bizarre house filled with an astounding array of collections.

The ancient tomb acts as a calendar that keeps perfect time.

An Irish church where an 800-year-old mummy is reaching out of his coffin as if to shake hands.

The gorgeous library at Trinity College is home to remarkable treasures of Irish identity.

The modern birthplace of Irish freedom.

Iceland's largest lagoon is home to stunning multi-colored icebergs.

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.

Medicinal spa created with the discharge from a geothermal energy plant.

This beautiful, historic church looks like something out of a simpler past.

The picturesque birthplace of Icelandic democracy is drenched in the blood of its brutal past.

An encyclopedic collection of mammal penises.

One of the tallest buildings in Iceland, and perhaps, the most visually impressive.

A series of waterfalls so massive they straddle the border of Argentina and Brazil.

An art museum with an eclectic design and a scandalous past.

A king's pet, a pope's gift, an artist's inspiration, and a tragic shipwreck.

One of the most beautiful bookstores in the world hides a neo-Gothic interior behind an art nouveau facade.

A graceful natural rock formation stands astride the point where the Gulf of California meets the Pacific Ocean.

The Battle of Atlanta in 360 degrees, featuring a corpse that bears a striking resemblance to Clark Gable.