Inside this mountain lies several kilometers of breathtaking caves.

Walk 2,300 feet above the ground on the "Troll's Tongue."

The largest of London's subterranean rivers and once a mad, bad center of London life.

The ghostly remains of an accidental sea.

This "preacher's pulpit" once used for pagan ritual, is home to a new, and dangerous ritual.

The last of its kind in the country.

Upside-down trees serve as natural flower pots in this hanging garden of Alaska.

A metal tree emits eerie, melodic notes on top of Lancashire's Crown Point.

Virgil's many-mouthed gateway to the underworld.

A stunning silver lining to global warming's gloomy cloud.

These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

This sword isn't waiting for a king to pull it out. It was placed there by a saint.

A tunnel of hardened lava along Maui’s famous Road to Hana.

Hundreds of hidden passageways weave through the old quarters of Lyon.

The world's second largest salt mine, said to have been discovered by Alexander the Great's horse.

A religious-themed park filled with folk art to bring biblical stories to life that fell into disrepair and now being restored.

Man-made cave with enigmatic carvings, some say made by the Knights Templar themselves.

Underneath this small church are 15,000 bones, arranged by an obsessive academic.

This small chapel is built of human bones and decorated with a golden skeleton.

Abandoned concert venue legendary for its rock counterculture.

A house of bones — but nobody is quite sure to whom they belong.

Beyond being incredibly fun, not much can be said about the Hole with certitude.

Thousands of exquisitely carved stone water storage wells, hundreds of stone steps fill this exquisite medieval stepwell.

Megachurch turned Catholic church comprised of 10,000 glass panes.