A celebrity from the late Cretaceous period.

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

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

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

This sandwich shop has a century-old elevator behind a sheet of plexiglass.

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

The only Civil War battle in Washington D.C. took place near this highest natural point in the city.

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.

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

The privately owned cemetery that holds room for Washington's finest when they step down from life.

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

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

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

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

You could be standing at the site of one of D.C.'s most fatal tragedies and not even know it.

D.C.'s first central library was born out of a chance encounter with the philanthropist whose name it bears.

Housing the National Women's Party since 1929, this historic house is now a monument to the fight for gender equality.

The oldest continuously operating fish market in the United States.

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

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

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

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

The national park was once a plantation estate.