Mother Nature outdoes herself with this stunningly beautiful set of caves carved into marble.

A massive underwater sinkhole made famous by Jacques Cousteau.

The mysterious wonder of an underground waterfall is illuminated by multicolored lights.

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

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

Wild lights and sound effects turn a walk through some caves into an eye-popping 1960s tourist attraction.

A lovers' beach, tucked below the surface of the island, provides a safe haven for romance.

A modernist landscape of water and sci-fi concrete.

This natural wonder drops 16 stories straight down through rare flora and streaming waterfalls.

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

The world's longest known cave system.

360° views of the city plus five floors of optical illusions and hands-on science.

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

A submerged archeological park still holds the wonders of a Roman Sodom.

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

The most iconic structure in Scotland has been home to bishops, colonels, and Sean Connery characters.

A beautiful 62-acre Catholic shrine and botanical garden.

The hike through a narrow river flanked by the 2,000-foot-tall walls of Zion Canyon is one of the best in the world.

The intricate stonework of Machu Picchu serves as a testament to Incan engineers.

Enormous lake at the bottom of a unique cave system.

The ancient dwellings of the Puebloans, located in the cliffs of majestic mesas.

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

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

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.