This mysterious structure by the Washington Monument is a flood barrier designed to protect the White House against rising waters.

Daredevil repair workers can worm their way out the access hatch, loop ropes over the apex and rappel down the monument.

The oldest building in the District of Columbia was preserved because of a mistaken connection to George Washington.

The discovery site of the "Capitalsaurus," the official dinosaur of Washington, D.C.

The national park was once a plantation estate.

The towering figures were created from the cemetery's fallen old-growth trees.

An overlooked oasis of quiet on the grounds of Washington's monumental basilica.

Cold War intelligence agents monitored communist embassies from an attic in a former pigeon coop.

A rotating cast of guys and dolls in front of a Washington, D.C. building.

A celebrity from the late Cretaceous period.

These recently uncovered walls are all that's left of Washington, D.C's first defense contractor.

A much-maligned monument to teetotalism.

It was once the base for a series of domestic terror attacks.

There’s a hidden military meme engraved on the World War II Memorial.

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

People escaping slavery may have hidden inside a corpse vault.

A traveling postal dog covered 48 states and more than 140,000 miles, and he lives on as taxidermy, patched up with a rabbit's foot and a pig's ear.

Light bulbs, scrap wood, and tinfoil comprise this homemade throne of the gods.

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

A Spiritualist house of worship where believers communicate with the dead in the spirit world.

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

The most decorated dog of World War I is preserved in the Smithsonian.

A lovely aquatic park built by a one-armed Civil War veteran who made a fortune from lotuses.

A derelict bit of transportation infrastructure hidden in the woods.