Largest man-made excavation on earth!

An earthwork sculpture built by Robert Smithson in the Great Salt Lake only reemerges during drought.

An organ located deep within a cave, whose "pipes" are the geological features of the cave itself.

The narrowest house in America is seven feet of pure spite.

This rare triple-jurisdiction drawbridge passes through Virginia, Maryland, and Washington, D.C.

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

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

Government testing at the asylum briefly explored using marijuana as a "truth serum" on Nazi prisoners of war.

A little marble compass above George Washington's (empty) tomb in the Capitol marks where D.C.'s four quadrants intersect.

Corn-inthian columns with a uniquely American take on neoclassical architecture.

"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”

The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

A 1,100-ton train fell through the floor in 1953. Workers got it patched up in just 72 hours.

One critic described it as "middle Mussolini, early Ramses, and late Neiman-Marcus." Another called it an architectural "natural disaster."

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

The actual model from the original "Star Trek" series is now on display at the National Air and Space Museum.

Washington's iconic carousel has a nice piece of Civil Rights history.

The remains of Martha, the last known passenger pigeon, serve as a tool to educate about conservation.

A muscular Art Deco monument represents the struggle between regulators and unbridled markets.

Home to items never before seen by the public.

A heroic pigeon that, through a barrage of gunfire, delivered a message that saved over 100 lives in World War I.

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

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