An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

The only visible reminders of an elaborate engineering project that kept the tower from reaching the tipping point.

Ancient snack bar of the Roman Empire re-opened.

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

The world's most active stratovolcano is almost always erupting.

The last moments of the ill-fated Pompeiians, frozen forever in plaster.

The spot in the Mile-High City that's exactly 5,280 feet above sea level (at least for now).

One of the greatest heroes of the Titanic tragedy came from humble beginnings to this beautiful home.

A stark reminder of the huge nuclear weapons plant that once stood on the site and its effect on the surrounding area.

A downtown park built on top of unclaimed graves.

The highest paved road in North America winds its way up Mount Blue Sky in Colorado to an altitude of 14,130 feet above sea level.

By day this New Orleans square is for pedestrians, but by night it is a kingdom of kittens.

The Pentagon sits on a former island that was in Virginia at low tide and D.C. at high tide.

This isn't your average roadway—it's actually a National Park and a transportation pioneer.

The semi-reclaimed ruins of this canal were once part of an ambitious plan to reroute the mighty potomac.

A secretive collection featuring objects that were designed specifically for U.S. intelligence operations.

The world's tallest doric column is this Ohio monument to a naval victory in the War of 1812.

Ghost stories surrounded this “castle,” the only structure from a huge mansion that was never built.

This spectacular public market has got meat, fish, vegetables, baked goods, and enough kielbasa for a lifetime.

These beautiful wineries were at the center of Lake Erie's "grape belt" back when the Midwest dominated American wine production.

The only presidential monument to display a late POTUS's actual casket.

Landmark dedicated to the plight of the American Indian in the 21st century.