An abandoned railroad tunnel transformed into a creepy path for hikers and cyclists.

This quarter-mile tunnel under the Bluff in Pittsburgh bends at a 45-degree angle.

An equine tribute in a charming Southern city that calls itself "The Horse Capital of the World."

Three courses accompanied by Christian rock.

Veritable museum of 20th century country music history.

This environmental engineering marvel keeps salt water from the Elizabeth River.

Long buried under the 20th-century landscape, this lift lock of the Alexandria Canal is the lone remnant of an ambitious early American transportation project.

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

Herbert Hoover's "Brown House" rural presidential retreat was a PR disaster amid the start of the Great Depression.

An out-of-place Air Force radar dome is all that denotes this park as a former missile launch site.

Hidden for 165 years inside a building designed by Thomas Jefferson are the remains of what may be America's first educational chemistry lab.

A double agent hid information for the Soviet Union beneath this bridge for more than 20 years.

This museum explores the grim reality of life in the earliest British colonies in America.

This cannonball stands as a reminder of the final act of a fleeting governor amidst a revolution.

This grove was planted more than a century ago by the War Mothers of World War I.

First built around 1848, this bridge was destroyed by arson, but original timbers are included in the replacement.

One of the earliest and best-preserved buildings in Charles County, Maryland.

Built in 1815, the stabilized ruins of the grist mill and sawmill are now gated, but interpretive displays on the features and functions of the mill preserve its history.

Washed out by the Johnstown Flood in 1889, the surviving portion of the original two-span bridge now resides at Fishing Creek.

A rural colonial settlement that contains the remains of a store and cooper.

One of the oldest standing structures in Montgomery County, Maryland.

The oldest surviving one-room schoolhouse in Montgomery County, Maryland.

The headquarters of the Audubon Naturalist Society is a small slice of heaven for nature lovers.

A state park that once served as an important river crossing, and now offers protected lands for endangered plants and animals.