Limestone caves with spectacular showcase chambers.

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

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

An abandoned facility that has become a place of ritualistic satanic worship, or so rumor has it.

This secluded aqua waterfall in the Grand Canyon is the perfect swimming hole, and the Havasupai tribe's fiercely protected natural wonder.

Dramatic river bend surrounds a natural red-rocked pedestal.

Most-visited and most-photographed slot canyon in the American Southwest.

Rippling sand dunes frozen in the Arizona rock.

A once-glowing grotto at the bottom of Waikapalae Wet Cave.

A beach made of semi-precious green sand on the Big Island of Hawai'i.

Hide from the tourist hordes in these relatively unknown, and unguarded lava caves.

Race through the maze, get your name on the sign.

The remains of a failed subway in upstate New York is now an underground graffiti bunker.

A bar, burger joint, and arcade housed in a late 19th-century church building.

A bronze ballot box stands at the spot where Susan B. Anthony voted illegally.

The floor-to-ceiling collection in an old factory is a unique alternative to stuffy galleries.

An old dining hall now serves up ghost stories for lusty locals.

The gravestone of the suffragist leader attracted extra visitors during the 2016 election.

A plaque marks a former safe house on the secret route, now hidden next to a McDonald's.

A century-old network of freight tunnels underneath the city's historic business district.

To find the "unicorns of the deep" you've got to go downstairs.

These abandoned grain silos and tunnels are an urban exploration dream.

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

A giant bronze coil of feces is both a tribute to doggie defecation and a reminder to pick it up.