A French chateau, failed utopian community, odd museum, and Stonehenge replica in rural Washington State.

Home of Howard Hughes' Spruce Goose.

The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

This sandstone mansion was the home of two of Oregon's most important pioneers.

A peculiar mansion built by the troubled heir to the Winchester Rifle Company fortune.

A museum on the history of Paris, including a set of Napoleon's toiletries, Proust's room, and relics of the Revolution.

The prototype for all Nazi concentration camps.

An incarceration museum on the spot of possibly the oldest prison in England.

Scattered throughout London are ancient remnants of the city's former bounding wall.

The doodles of a genius line the walls of a secret room under the Medici Chapel, where they were hidden for nearly 500 years.

The Emperor Tiberius' palace of debauchery.

Five candy-colored villages on the Italian Riviera.

Built to repay the sins of the father, this church is home to one of the most valuable works of Western art.

Europe’s oldest natural human mummy.

Fortified medieval tower house with lovely roof garden.

The Pope’s secret escape route.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

Erotic art covers the walls of the largest ancient pleasure palace.

Ancient snack bar of the Roman Empire re-opened.

Go inside to really appreciate this 500-year-old technological masterpiece.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

The seawater playground of gilded-era San Francisco burned to the ground in 1966.

These delicate buggers are some of the rarest fossils that have bubbled up from the Tar Pits.