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

This quirky little private museum is home to more than a thousand stones, carved by nature, that resemble human faces.

A neighborhood park in the city of Cáceres hides what some believe are revolutionary examples of Neanderthal cave art.

A natural dome made of volcanic glass in Inyo National Forest.

The eroded core of an ancient volcano is now accessible by a short hike.

Eons ago, a rushing lava river formed this pristine passage through solid rock.

The building features one of the last remaining free-floating spiral grand staircases in the U.S.

A mass grave for victims of an airline disaster that led to much-needed safety improvements for commercial flights.

This modern French ossuary contains the jumbled bones of 130,000 WWI soldiers.

A swath of France so devastated by war it is still forbidden to go there.

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

Many people don't realize there’s a lot going on in—and below—the world’s tallest triumphal arch.

This Gothic castle-like portal is a testament to the amazing craftsmanship of the navvies who built the railway tunnel.

Rare recess caves fill this unbelievably beautiful Ohio park.

Oregon's unexpected high desert, displaying layers of time in a colorful show.

This great stone guardian of the forest is a relic from the land's volatile volcanic past.

Bridges, terraces, and replicas of historic structures all built of rock and petrified wood by one man.

This futuristically sylvan church is a glass-enclosed marvel of modern architecture.