Intensely colorful calcite pools in Southern China.

Snow-capped peaks, colorful waterfalls and stunning vistas on the edge of the Himalayas.

Turnip-shaped island just off the Michigan shore in Lake Huron.

A roaring, awe-inspiring double cascade of icy water flows through woodland Oregon like something out of a Tolkien novel.

This extraordinary geological oddity resembles human-made pavement.

The deepest lake in the United States, and once the site of epic destruction that lives on in legends.

Marvel of green building techniques that sings with the wind.

A beautiful geyser under environmental threat.

Roman ruins top white travertine terraces formed by ancient hot springs.

Half of a waterfall goes into the river while the other half seems to disappear.

The world's longest known cave system.

Ancient paintings known as the "Sistine Chapel of Cave Art."

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."

Hiding the longest subterranean river known in the world.

Monasteries on rock pillars, once accessible only by frayed ropes.

A spectacular cave and a beguiling lake.

A gargantuan crystal kingdom.

Beautiful carved tunnels.

Limestone grottoes turns everyday objects into stone.

This "preacher's pulpit" once used for pagan ritual, is home to a new, and dangerous ritual.

A thin pane of glass keeps visitors from sliding off this futuristic skywalk.

Walk 2,300 feet above the ground on the "Troll's Tongue."

This giant boulder is pinched over a 1000-meter abyss by two mountains.