Known for the tunnel that leads visitors to the water.

This electric green sulphur lake in New Zealand looks like a cartoonish radioactive dump site.

A glacier pocked with ice caves and surrounded by rainforests.

A complex of six forts embedded in the Rajasthan hills.

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

This otherworldly palace-mausoleum complex is the stuff eternal dreams are made of.

More than half of this Indian palace is drowned but it looks all the more spectacular for it.

Rock-cut cave complex filled with ancient Buddhist artwork.

The remains of what was, not so long ago, the world's largest city.

This complex of vertically-excavated Buddhist, Jain, and Hindu monolithic temples in India's Maharashtra providence illustrates a spirit of tolerance characteristic of ancient India.

This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

A palatial sculpture park was built illegally and in secret by a self-taught artist over 36 years.

An enchanting monastery in the Spanish countryside appears lifted from a fantasy tale.

Visitors can walk over the largest concentration of Columbian mammoths to have died from one event.

This memorial commemorating fallen Swiss Guards of the French Revolution has been called the saddest stone in the world.

Last residence of H.P. Lovecraft featured in his story "The Haunter of the Dark."

This Irish show cave is said to have been the site of a Viking massacre that claimed 1,000 lives.

Natural stone arches form a walkable "cathedral" at low tide, then disappear entirely when the water returns.

Once an ambitious nature spa, these forest ruins now provide a serene monument to failure.

A mecca for India's Jains.

Sweden's first nuclear reactor, R1, can be visited 25 meters underground.

This legendary city has been a tourist attraction for centuries.

Two geological phenomena in one precarious locale: a towering stone window, and a deep blue void.

A limestone wonderland of water and subterranean secrets.