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

The hidden offices underneath Memorial Bridge have been locked up since 1976.

Obscure Freemasons still live in D.C.’s largest private residence.

Art deco landmark on the outskirts of Washington, D.C.

A much-maligned monument to teetotalism.

The unique traffic pattern named for an influential urban planner is also known as the Pedestrian Scramble.

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

A three-story tower of books about Abraham Lincoln is one of the more unusual monuments to the president.

Members of Congress have traveled between the buildings on Capitol Hill for a century hidden from tourists, press, and storm clouds.

A historic ruin just 20 feet away from a busy highway onramp.

An unassuming, powerful monument north of the U.S. Capitol bears witness to the resilience of Japanese Americans during a time of grave injustice.

An Industrial Revolution-era public work that purified water using nothing but sand.

A celebrity from the late Cretaceous period.

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

Once the supplier for noteworthy projects like the National Cathedral, this old brickworks now lies abandoned.

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

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.

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

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

This stunning blue and gold room changed cities twice before becoming part of the Smithsonian.

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.

The oldest continuously operating fish market in the United States.

There's a crumbling old mansion inside the Smithsonian National Zoo.

Four bald cypress trees in Lafayette Square, across from the White House, mark the southern edge of a 100,000 year old cypress swamp.