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

A stunning, two-tiered waterfall made famous by poets, artists and the legend of a ghost dog.

A beautiful geyser under environmental threat.

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

Paris' linguistic guide to the international language of, yes, love.

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

Four glass boxes hover over 1,000 feet in the air to give visitors an unparalleled view of Chicago.

Northern Ireland's Brobdingnagian stepping stones.

Ruins of a German Castle on top of a towering stone pillar.

There is no trying to fall in love with this Star Wars fountain, you just do or do not.

A semi-scholarly approach to sex.

Towering basalt columns, a creation of fire and ice.

A giant Troll lurks under one of Seattle's oldest bridges.

The installation’s official name, “You Who Are Getting Obliterated in the Dancing Swarm of Fireflies,” says it all.

America's most famous piece of self-built architecture.

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

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

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

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

Dramatic river bend surrounds a natural red-rocked pedestal.

The fairy tale castle of the "fairy-tale king."

A collision of human error and natural geothermal pressure created this rainbow-colored geologic wonder.

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