This mountain fissure hides a secret path to a secluded mossy grotto within the rock itself.

The world's longest known cave system.

Four glass boxes hover over 1,000 feet in the air to give visitors an unparalleled view of Chicago.

The hike through a narrow river flanked by the 2,000-foot-tall walls of Zion Canyon is one of the best in the world.

A house carved into a rock in the 1940s.

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

Dramatic river bend surrounds a natural red-rocked pedestal.

Bizarre lunar-like landscape with thousands of large stone "hoodoos."

Legends surround this jagged rock formation in the New Mexico desert.

The ancient dwellings of the Puebloans, located in the cliffs of majestic mesas.

The marker offers a rare chance to be in four U.S. states at once, though there's some controversy over its geographic accuracy.

The highest point in Tennessee, Clingmans Dome bears witness to the ravages of one type of insect.

Gaudí's famously flamboyant architectural style is on full display in this sprawling park and garden.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

One man's fulfilled dream of living on an artificial island made of recycled plastic bottles.

Beach instrument turned bed and breakfast.

A six-ton wedding present for the King of the Scots.

A red sandstone freestanding monolith rises out of the sea in northern Scotland.

The most loyal of little dogs, or a Victorian era publicity stunt?

Possibly the oldest of Britain's ancient henges is also one of its most stark.

The final resting place of soldiers' loyal canine companions.

The true age of this ancient stone circle is unknown, but its stunning vista isn't in question.

Largest lake in Great Britain measured by surface area.