This canyon is named for a unique geologic formation where the river vanishes underground near the mouth of the canyon and reappears farther down.

Native Americans built this circle of stones, which was used to predict astronomical events.

This museum details the history of a local miracle: The return of the critically endangered black-footed ferret, which was once believed to be extinct.

At the time of its completion, it was the tallest dam in the world.

An unusual geological feature towers over the remains of a dark episode in United States history.

Derelict buildings are all that remain of a complex at the site of one of the worst mining disasters in Montana history.

Around half a million years ago, these magnificent columnar basalt cliffs were formed from cooling lava.

No Gordon Lightfoot song, only a barebones monument to greed.

The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.

One of nature's most well-scheduled phenomenons resides in Wyoming's Yellowstone National Park.

An architect's childhood fantasy treehouse made real.

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

The hallucinogenic center of the ancient world.

The ruins of an abandoned medieval city where the last Byzantine emperor was crowned.

This legendary city has been a tourist attraction for centuries.

Step inside an ancient beehive tomb built for a mysterious cult hero.

This slumbering beast honors the Bavarian soldiers who died in Greece during a 19th-century typhoid outbreak.

This 15th-century island fortress later served as home to the executioners no one wanted to live near.

Legend says this sacred tree was a gift from goddess Athena.

An ancient Greek monument and a Byzantine church coexist inside this cave.

Heralded as the world's first theater, these ruins are often overlooked as they reside in the shadows of the Acropolis.

The ruins of an ancient fortification wall sit in front of and underneath this boutique hotel.

This mystical orb covered in strange symbols is thought to have been used in ancient Greek magical rituals.

The oldest cemetery of modern-day Athens blends the modern and ancient worlds.