Mysterious craters may have been home to an ancient cult.

An extraordinary natural landscape of mountains, waterfalls, and rocky cliffs riddled with tunnels.

Ruined 11th century abbey built by King David I in Edinburgh, Scotland.

Legacy of the earthquake that nearly wiped Lisbon off the face of the Earth.

The ruins of an ancient island city built atop a coral reef in the South Pacific are spectacular and strange enough to have inspired H.P. Lovecraft.

A mysterious pre-Incan ruin believed to have miraculous healing powers.

Abandoned city hidden from the world by tropical forest and isolation on the coast of Kenya.

Tropical paradise strewn with hundreds of World War II wrecks.

The remains of a sunken military ship wreck visible through clear water.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

A prehistoric landscape hidden in the cliffs of Maha'ulepu.

The ravine where Jesse James supposedly performed an impossible jump to evade capture.

This natural Icelandic fissure allows divers to swim right between two volatile tectonic plates.

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

This dramatic sea spire is home to the world's rarest insect and is part of the lost continent of Zealandia.

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

A humid oasis combines recreation with exploration in a unique underwater cavern.

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

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

A popular swimming hole that hides one of America's most deadly diving spots.

Civil War dumping ground for the bodies of terrorized Union sympathizers.