A city built from mud and known as the "Manhattan of the desert."

The headquarters of the Mahanikai school of Buddhism, Thailand's largest monastic order.

Holy Hindu shrine in an enormous, monkey-filled limestone cavern.

An ancient stone fortress considered by many to be the eighth wonder of the world.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

A marvel of Byzantine engineering under the streets of Istanbul.

A spectacular cave and a beguiling lake.

The Avenue of Mausolea is a series of grand palatial tombs.

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

Sacred Indian temple that’s all monkey business.

The world's second-largest excavated hole.

An Ethiopian "Jerusalem" featuring a church hewn out of a single rock.

The largest cave shaft in the world is home to thousands of birds and popular with BASE jumpers.