Disks are rolled out onto the south lawn to absorb the impact of Marine One's wheels like giant coffee coasters.

An echo chamber with a waterfall wrapped around its base at the Embassy of Canada in Washington, D.C.

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.

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

The word "Watergate" will forever be associated with the infamous scandal in 1972, but this fountain is famous in its own right.

This map, on display in the Library of Congress, and one of only seven known to exist, is the very first map of the United States.

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

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

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

A derelict bit of infrastructure from the canal that once ran through D.C. is landlocked in the heart of the city.

The "lovely red Vega" of the legendary record-settling pilot.

The oldest continuously operating fish market in the United States.

A super-sized promotional trick that is now a D.C. landmark.

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 remnants of a wooden-hulled fleet built and abandoned in WWI, now home to a thriving ecosystem.

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

The 72-foot giant escaped confinement in a large patch of mulch only to be reburied in the sandy shores of the Potomac River.

A mammoth pair of Civil War artillery guns abandoned out in the forest.

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

The ruins of an 18th-century mansion that was once named "Mistake" by the man who had it built.

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

This rudder is one of the few remnants of a shipyard that once operated on the banks of the Potomac River.

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