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

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

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.

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

The most accurate timepiece in the world.

A hoard of sky catalogs, astrophysical journals, even the works of Galileo and Copernicus.

A celebrity from the late Cretaceous period.

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

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

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

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

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

The site of the climactic scene from the classic horror film is now a historic landmark.

The corporeal remains of Napoleon III's dentist rest under this 15-story obelisk.

This sculpture of a fisherman spent six years at the bottom of the Schuylkill River.

One of Philadelphia's largest murals resides hidden under this bridge.

South Philadelphia is home to Keith Haring's only surviving in situ collaborative public mural.

In the early 1800s, surgery was also a show.

This little-known Revolutionary War fort is supposedly haunted by the ghosts of its past.

Ant sculptures march up the side of what used to be an iconic spot in punk rock lore.

A clone-tree in Washington Square Park sprung from seeds that went to the Moon and back.

World's first "penitentiary," meant to be humane, drove men insane.