The school tested mustard gas for the U.S. Army during World War I.

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

The little-known, 300-year history of the area includes former lives as a bustling tobacco port, parking lot, and industrial dump.

While most wind tunnels test scale models, the "Cave of Winds" was large enough for actual airplanes.

Before founding the American Red Cross, Clara Barton had a tremendous humanitarian impact by locating thousands of missing soldiers.

A heroic pigeon that, through a barrage of gunfire, delivered a message that saved over 100 lives in World War I.

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

Architecture lovers won’t stop touching the National Gallery's 19.5 degree marble prow.

There’s a special club house at the top for the bell ringers.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

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

This imposing Masonic temple a mile from the White House was the first public library in Washington, D.C.

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

A celebrity from the late Cretaceous period.

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

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

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

The first presidential bomb shelter was located in an old vault under the Treasury, connected to the White House via tunnel.

A little marble compass above George Washington's (empty) tomb in the Capitol marks where D.C.'s four quadrants intersect.

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

Decades before the scandal, this staircase on the river was a literal "water gate."

Perhaps the most scandalous statue of America's first president.

The most accurate timepiece in the world.

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