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

This strange seaside tower got its name from a wealthy eccentric who would hide coins among its stones.

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

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

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

A "gravitational anomaly" located in the redwood forests just outside of Santa Cruz, California.

The world's longest known cave system.

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

Two thousand artifacts from around the world collected by one woman who loved to travel.

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

A lush green oasis in the middle of Chicago.

The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.

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

This 4,000-foot-diameter hole is touted as "the most well known, best preserved meteorite crater on Earth."

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

This upscale streetwear store is hidden behind a fake Snapple machine in the back of a deli.

Read a book next to a Midtown waterfall.

A self-built castle made by a mysterious man.

A heavenly experience, right here on Terra Firma.

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

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

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

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

A castle that was built stone by stone by an eccentric journalist.