The U.S. Fish Commission’s effort to mass produce carp.

The romance of early commercial flight still fills this Art Deco destination.

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

This bug-eyed water wheel uses the power of the Sun to clean up Baltimore Harbor.

The front yard of an Upper East Side town house hides the last fragment of one of New York's most famous theatres.

35,000 grape hyacinths flow like water down a garden hill.

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

The school tested mustard gas for the U.S. Army during World War I.

There’s a special club house at the top for the bell ringers.

Walk a trail through a historic Quaker town that outlawed slavery in 1777 and was a major waypoint on the Underground Railroad.

An absolutely beautiful 200-foot stained glass installation traces the evolution of knowledge.

The walls of the iconic dome are hollow and have a secret stairway.

A century-old network of freight tunnels underneath the city's historic business district.

Every drop of D.C. tap water flows through this old waterworks.

Where the U.S. government kept its actual treasure, before Fort Knox.

The monument's pointy aluminum tip has been melted down by repeated lightning strikes.

Seemingly unremarkable items like empty sushi trays, chef hats, and freshness stickers are being preserved so future generations can look back on this beloved cultural import.

This seemingly out-of-place gas station by the Watergate hotel was once described as the most expensive gas station in the world.

An atomic bomb-proof strongbox protects the U.S. Constitution from terrorists and thieves.

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

The 320-year-old tree that greeted General Lafayette and rerouted an interstate highway.

While most wind tunnels test scale models, the "Cave of Winds" was large enough for actual airplanes.

This fort down the Potomac from Washington, D.C. was once the only defensive fort protecting the capital.

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