Jesus' footprints and the first catacombs in the world.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

The final resting place of the poets Shelley and Keats.

The skull of the patron saint of lovers lies in the Basilica di Santa Maria in Cosmedin—maybe.

2300-year-old Roman road connecting the empire.

A master of trompe-l'oeil creates a stunning, tourist-fooling dome out of a flat ceiling.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

One of the world’s largest collections of petroglyphs records 2,000 years of human activity.

The fifth-longest natural arch in the world, and the longest outside China.

Despite its name, this route in southern Utah offers a heavenly day trip.

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

These faux-cliff dwellings allow visitors to climb around on a replica archaeological site.

This spectacular track has run continuously since 1882.

The highest bridge in America was built for an almost unnervingly small amount of money.

Strange red rock formations that have been attributed to a higher power for hundreds of years.

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

These ruins of a pre-Columbian cliff village are among the most beautiful and well preserved in New Mexico.

A multi-storied adobe complex has been inhabited for more than a thousand years.

These aggressively sustainable art homes look like something out of 1970's science fiction.

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

Explore the largest pure gypsum deposit in the world, and go dune sledding while you're at it.

This historic hotel on Bathhouse Row in Hot Springs National Park has a storied history of gangsters and baseball greats.