In the basement of the U.S. Capitol Building is a hidden storage room full of ornate floor tiles leftover from the 1850s.

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

Those with a sharp eye can find the hidden image of a German Shepherd on the memorial's Mural Wall.

This poor little statue is the most vandalized memorial in Washington.

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

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

The span is crooked and made from two older recycled bridges.

An easily overlooked memorial to a Ukrainian famine-genocide that killed over 4 million people.

A museum cafe showcases Native American dishes and indigenous ingredients from across the Western Hemisphere.

A clone of a cloned tree that was so important it allegedly had its own guards.

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

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.

Home to a vast and influential collection of Shakespeareana.

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

A computer bug so primitive it was an actual insect.

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

A haunting tribute to pioneering photographer Clover Adams.

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

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

An obscure public service from the U.S. government that redeems burnt, moldy, and soiled old greenbacks.

This leafy estate is worth $18 million and is so grand it has two mailing addresses.

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

When it opened during the final years of the embargo, all the drinks and cigars were free.

Largest Roman Catholic church in North America.