A massive cave complex boasts some of the clearest water (and most charming bats) in the world.

An ethereal glowing blue ocean cave in Croatia.

Tubing through a cave lit with glowworms is a magical underground experience.

Discovered by cows, these upstate New York show caves are presented with a healthy dose of psychedelic folk art.

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

This natural marble cave is one of only five national monuments in Oregon.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

Travel through the wholesome amusements of yesteryear at this charming miniature village.

This massive home was constructed by one of the greatest conservationists in Colorado history.

In the middle of a suburban Denver neighborhood is a cave that once provided solace for a prominent Ute chief and his family.

A cave's rare beauty held a cavernous secret hidden underground.

The Hanshin Expressway goes right through its 5th, 6th, and 7th floors.

This ancient burial site was once thought to be the home of a race distinctive to Ainu folklore.

These wondrously illuminated caverns extend under the mountains of the Fukushima Prefecture.

Approximately 300 million years ago, a massive coral reef existed in what is now the largest limestone cave in Japan.

Designed by Buckminster Fuller, this relic of the 67 Expo survived fire and ice.

An off-the-beaten path cave with an underground lake.

The largest cave in Northern Europe.

One in a pair of subterranean caves created by an ancient eruption of Mount Fuji.

Perhaps the most spectacular grotto along the Algarve coastline.

Chiseled out of a sandstone cliff, one farmer's unfinished art project lies in ruins.

This 13th-century monastery chiseled into the side of a mountain is equipped with a bridge that can be lifted to protect against raids.

A series of caves, tunnels, and wells filled with fascinating figures and symbols.