Ha Long Bay's "Cave of Surprises" houses a popular phallic rock formation.

An ethereal glowing blue ocean cave in Croatia.

Behind this beachfront shop lies a spectacular, colorful sea cave with an equally colorful past.

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

This series of Armenian caves held the oldest brain, shoe, and winery in the world.

One of the most remarkable caves in all of Iceland was created by a once raging river of lava.

An extraordinary "City of Caves" carved from solid stone.

This Navarrese cave's history of witchcraft and pagan ritual is not just embraced, but recreated each year.

Stone Age solar temple is aligned with the sun on each equinox and solstice.

Gladiators battle and chariots race daily in the ruins of a once-great Roman city.

The stones of this Incan wall complex fit together so well you couldn't fit a piece of paper between some of them.

Everything from a plane to your own two feet are needed to reach this jungle resort.

A Surreal flight of architectural fancy in Vietnam's Central Highlands.

These homes carved into the mountains seem to defy the laws of nature.

Live a night as a Byzantine King, with a minibar and private jacuzzi.

An isolated village literally built into the crevices and rock formations of a mountain.

Underground monastery and fortress.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

An Amazonian hotel is the ultimate treehouse and provides stilt based accommodation.

This egg-shaped treehouse was inspired by "sport sleeping."

Keshwa chaca, among the last handwoven Incan bridges, crosses Apurimac Canyon in Peru.

Nazca pilgrimage site covering 370 acres is a popular stop for archeologists and looters.

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

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