This coastal cavern is associated with the Hawaiian god of creation.

Generations of rockhounds got their start at this beloved, historic mine.

An underground wonderland in Iowa.

This small cave in the center of Isla Colon doubles as a sanctuary.

The spectacular subterranean collection includes dazzling minerals, including a piece of "kryptonite."

Kakadu National Park is known for its stunning beauty and its rock art, which represents the longest historical records of any continuous civilization on Earth.

An Industrial Revolution-era public work that purified water using nothing but sand.

Canada's subterranean Cold War museum has doubled as a movie set.

If there's another drive-through golf-cart bar in a bat-filled cave on top of a mountain, we don't know about it.

You'll need cowboy boots to trek to this old-fashioned saloon nestled in a giant natural cave.

This nondescript tunnel has been featured in a number of iconic movies, from “Roger Rabbit” to “Back to the Future.”

A gargantuan crystal kingdom.

Peek into a cave of giant, glimmering crystals.

This Cold War–era underground city includes offices for Mao Zedong and Lin Biao.

This Armenian monastery was carved out of cliffs and named after the spear used to stab Jesus.

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

She asked for a potato cellar and over the next 23 years he gave her a network of holy caves.

Right up until the 20th century this network of man-made caves was the biggest village in Eastern Armenia.

Located in the remote Monitor Valley, this travertine bowl contains a scalding pool of water.

Narrow slot canyons in a badland eroded into soft sedimentary rock provide an otherworldly landscape in this remote Nevada state park.

An errant donkey ushered in a new era of mining in Nevada.

This odd little home was built right into an African rock face.

An 18th-century hideout became a permanent and ongoing residence for one Lesotho tribe.

Long tunnels run under the ancient chariot racing circuit to the tower.