The largest natural travertine bridge in the world is nestled in a small canyon in central Arizona.

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

Eons ago, a rushing lava river formed this pristine passage through solid rock.

In a 345-million-year-old cave that's 200 feet underground, dinner awaits.

The largest cave in Northern Europe.

The stunningly blue waters of these beautiful caves also have a violent past.

This ancient cave is home to a museum, bar, and is filled with fluorescent minerals.

This forgotten chalk mine was reopened thanks to a massive effort from the local community.

Perhaps the most spectacular grotto along the Algarve coastline.

Even Henry David Thoreau found this glacial pothole irresistible.

This narrow canyon has been carved by the river over thousands of years into spectacular and curious forms.

Ruins of the elaborate house where the enigmatic costume designer threw glamorous parties for New York's theatrical elite.

Bizarre Cavern.

The 320-year-old tree that greeted General Lafayette and rerouted an interstate highway.

An isolated island home to one of the oldest English-speaking communities in the region and Maryland's state dessert.

This dig site outside D.C. is known for its exceptionally high density of baby dinosaur fossils and dinosaur eggs.

Eighteen miniature death-scene dioramas.

The country's oldest continually-operated mini golf course lies hidden in plain sight, eclipsed by one of D.C.'s most popular tourist attractions.

The sci-fi villain is a little-known inhabitant of the U.S. capital's largest cathedral.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

A clone of a cloned tree that was so important it allegedly had its own guards.

Giant acorn-shaped 19th century gazebo from which suburban Washingtonians gazed upon the original "silver" spring.

Live data is transmitted from the Atlantic coast to instantly recreate the ocean waves in this fountain at the NOAA headquarters.