Ride the subways of yesteryear.

Strange red rock formations that have been attributed to a higher power for hundreds of years.

It's not hard to see why the historic Peabody Conservatory of Music's library has been described as a "cathedral of books."

This natural wonder drops 16 stories straight down through rare flora and streaming waterfalls.

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

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

New York City's smallest piece of private property.

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

The world's longest known cave system.

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

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

A self-built castle made by a mysterious man.

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

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

Where manatees are the snowbird vacationers in these balmy Florida springs.

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

A heavenly experience, right here on Terra Firma.

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

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

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

A breathtaking array of ancient Pueblo structures and artifacts counterintuitively preserved by neglect.

Home to items never before seen by the public.

Towering basalt columns, a creation of fire and ice.

A beautiful geyser under environmental threat.