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

Daredevil repair workers can worm their way out the access hatch, loop ropes over the apex and rappel down the monument.

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

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

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

The Lincoln Memorial has inspired many, even though a word on the inscription appears to be spelled incorrectly.

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

The house where John Wilkes Booth conspired with his co-conspirators.

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

A tiny monument to the unsuccessful attempt by Thomas Jefferson to place the prime meridian in Washington.

An overlooked memorial honoring the local Washington residents who died in World War I.

Look closely and you’ll notice that the color changes a third of the way up the tower.

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

The U.S. Fish Commission’s effort to mass produce carp.

The only National Monument ever to go missing for nearly 50 years then resurface in a dump.

One of the most influential presidents in U.S. history wanted only this plain, elegant monument as his lasting memorial.

Carved initials which are nearly 500 years old preserve the deposed Queen's presence at Hampton Court Palace.

In 2013, historians discovered a hidden room devoted to the art of making royals' hot chocolate.

A gallery specifically dedicated to the botanical illustrations of a remarkable traveling lady.

A marine aquarium with an astounding variety of fish hides in the basement beneath the famous Palm House.

These docile dragons are ideal inhabitants for the conservatory greenhouses.

This beautiful deer park was built so King Charles I and his court could go hunting while escaping a deadly plague outbreak.

It is against the law to obstruct the view from the top of this English burial mound.

A commemorative plaque at the door pays tribute to an "American Patriot" known for his betrayal.