The beautiful 16th-century stepwell lies in the shadow of a famous fortress.

Curious cave that stays a cool 56 degrees all year long.

Partially collapsed lava tube and cave system complete with concert hall, underground pond, and unique albino crabs.

Massive series of 45 caves and over 51,000 sculptures carved from cliffsides in China.

A fifth-century temple improbably built into the side of a cliff.

This fresco-covered medieval monastery hangs from the side of a steep cliff.

148 feet long and in the middle of a verdant forest.

Part of one of the largest flooded cavities on the planet, and littered with prehistoric animal bones.

This ancient tunnel is one of the most important examples of Roman hydraulic engineering in Spain.

A spectrum of colors awaits visitors to one of Mexico's most gorgeous bodies of water.

A former travertine mine traces the geological history of this unique desert filled with crystalline blue pools.

The damming of the Colorado River buried an iconic geologic formation before record-breaking drought brought it back.

A beautiful red-faced slot canyon in northern New Mexico.

Wee (and not so wee) footprints that insects and reptiles of the Paleozoic Era left behind.

This towering volcanic plug stands out among the dramatic formations that dot this otherworldly desert valley.

This bird-shaped rock formation defies gravity, time, and classification.

The 7,000-year-old calcium carbonate formation creates a magnificent natural bridge.

Seemingly grown on some other world, these New Mexico rock formations look like a disused science fiction set.

A unique geological area spiked with wind-carved rocks that resemble teepees.

Explore the largest pure gypsum deposit in the world, and go dune sledding while you're at it.

Natural sonic phenomenon comes with a grisly legend of murder and blood.

A strange sandstone formation in the middle of nowhere presents an interesting climbing opportunity.

A natural arch offers panoramic views of Yangshuo's karst hills.

Unlike most arches, this geological oddity formed in a volcanic dike.