The world's longest known cave system.

148 feet long and in the middle of a verdant forest.

An apocalyptic seismic event creates a deadly chain reaction that ends with a 6-mile long lake developing within weeks.

An arsenic-filled pool in the Grand Canyon takes on the appearance of a giant, overflowing pumpkin.

Gorgeous cave resemble a winter wonderland.

This strange geological landscape erupts with sudden bursts of water.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

Mother Nature outdoes herself with this stunningly beautiful set of caves carved into marble.

Limestone caves with spectacular showcase chambers.

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

Home to the largest urban bat colony in North America, estimated at 1.5 million bats.

A massive sea stack famous for its appearance in The Goonies.

An emerald-green grotto just a short trip from Austin.

An abandoned Gold Coast-era mansion once intended for the exiled ruler of Albania, who never arrived.

An abandoned stretch of highway, deteriorating next to the operational one.

Crystal clear lake with a shoreline of travertine located in a Colorado canyon.

A rocky sea serpent hides caves, blowholes, and a spectacular natural bridge.

Beneath the mossy overgrowth covering this once-sprawling campus lies decades of history and the secrets that come with it.

An unrealized dream to wirelessly electrify the entire Earth.

A toxic ghost town sitting atop a massive coal fire.

A once grand hotel is being retaken by the wilderness it once capitalized on.

A mysterious air force base built to resemble a Long Island fishing village.

Protecting the future of some of the rarest and most misunderstood mammals of North America.

Collapsing castle on the Hudson river was once the storage site of military surplus business.