2300-year-old Roman road connecting the empire.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

The final resting place of the poets Shelley and Keats.

The underground sanctuary of a mysterious ancient cult.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

An alchemist's "magic door" stands in the middle of a Roman park.

This domed temple was built in a hidden cave mouth as a refuge for sinners seeking absolution.

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

On the Oregon coast there is a natural hole that seems to be draining the sea.

A museum of carnival attractions where you are invited to ride the rides.

These fantastically named natural pools look like something out of a Scottish myth.

A sulfuric hellscape in southern Italy.

A submerged archeological park still holds the wonders of a Roman Sodom.

This open air archeological site houses almost as much wildlife as it does classical religious artifacts.

A deadly ancient portal to the underworld.

An ancient city exhumed from the depths of the ocean.

The growing stones of Romania.

This massive subterranean fissure is known as the "underground Grand Canyon."

One of the largest, oldest, darkest, quietest motel rooms in the world.

A gargantuan crystal kingdom.

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

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.

Once thought to be the mysterious work of a witchy prophetess, a well with unusual properties turns objects into "stone".