Mile-long cave system near Great Smoky Mountains.

The world's longest known cave system.

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

A popular rock that acted as a waymarker for American pioneers.

This marker identifies the location where three states converge.

At the tallest peak in Colorado, an electrifying mix of thin air, lightning, and rivalry.

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.

A little-known hot spring haven right below the highway.

Paranormal experiences in this hotel's room 217 led Stephen King to write "The Shining."

The hippo that stands on this university campus was once a drunken flea market purchase.

A replica of the famous memorial statue at the Normandy American Cemetery hides in an office building in downtown D.C.

A hidden passageway now marks the site of an assassination attempt on Ronald Reagan that some say broke a 140-year-old curse.

An overlooked memorial honoring the local Washington residents who died in World War I.

A tiny monument to the unsuccessful attempt by Thomas Jefferson to place the prime meridian in Washington.

Look closely and you’ll notice that the color changes a third of the way up the tower.

The unique traffic pattern named for an influential urban planner is also known as the Pedestrian Scramble.

An underground blaze turned this village into a ghost town.

A mine fire has been burning under the deserted town since 1962, but this church is still going strong.

This former zinc operation has a 90-year toxic history.

A 21-foot-tall statue of a miner made of scrap metal stands in this West Virginia coal town.

A beautiful hidden gem in central West Virginia.

In an odd geographic anomaly this city borders two different states on opposite sides of town.

This claimant to the title of "smallest church in the 48 states" doubles as an equally tiny, quasi-post office.

Spectacular mineral formations fill these caves, which have been used for everything from smoking meat to stashing illicit liquor.