A hidden network of subterranean passageways lies tangled beneath Indianapolis' bustling City Market.

A sacred site for Native Americans surveyed by George Washington and owned by both King George III and Thomas Jefferson.

This geothermal hot spring hides beneath a slowly growing mineral dome.

An acoustic anomaly with a mysterious cause.

A modernist landscape of water and sci-fi concrete.

This knobby, drooping New Orleans oak is a favorite place both for climbing and for meeting giraffes.

One person's lost luggage is another person's treasure.

The largest self-built castle in the U.S., six decades in the making.

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

The world's longest known cave system.

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

A self-built castle made by a mysterious man.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

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

Seemingly grown on some other world, these New Mexico rock formations look like a disused science fiction set.

This abandoned asylum was once a state of the art facility before devolving into one of the most deadly mental institutions in American history.

Take a drunken tour inside the world's largest known geode.

This frequently-photographed waterfall takes only a short hike to visit.

Hollowed out cedar log tubs set in an ancient towering forest.

This modern antique shop is a museum for 1980's Saturday morning cartoon culture.

Where beautiful signs live on forever.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

The world's only international cryptozoology museum is host to an unrivaled collection of mysterious objects.

The inspiration for H.P. Lovecraft's horror stories and Batman's Arkham Asylum is now a condominium complex.