The vast, legendary catacombs hold secrets much stranger than stacked bones.

A psychedelic labyrinth on the San Francisco bay.

Only a handful of these gorgeous historic waterwheels remain but even those are in danger from local unrest.

A 1,500-year-old ghost settlement is slowly fading away.

Ruins of the Crusaders' headquarters.

Mysterious lost empire of the Silk Road.

A 1,500-year-old, 3-story subterranean city where ancient Persians took refuge in times of war.

An underground paleo-Christian burial site composed of the remains of several cemeteries and Basilicas.

The underground sanctuary of a mysterious ancient cult.

Rebuilt remains of a temple to Roman god Mithras.

The largest catacomb system in the world.

Possibly the most beautiful carillon in the world, set atop Iron Mountain in Florida.

Nestled between a college and a golf course, the garden is home to more than 2,500 rhododendrons.

The United States Capitol's former columns still stand.

Burial grounds for more than 1,000 bodies deep under the modern town of Rabat.

The palace of the King of Kings, burned by Alexander the Great.

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.

This giant volcanic crater holds its very own field of salt.

More than 100 monumental tombs carved into rock are preserved at this ancient Nabataean site in the Arabian Desert.

Rock-hewn monastery complex with hundreds of cave structures tucked away into the mountainside.

Underground monastery and fortress.

Ancient rock-hewn town that witnessed key events of Georgian history.

Village of slate houses, built on a white marble outcrop.

An unfinished tomb carved into a giant boulder in the Arabian Desert.