The Washington Monument is two colors by accident.
During the monument's construction in 1854, the Washington National Monument Society ran out of money. Construction was put on hold for 25 years, after which the federal government took the project over and completed it in 1884, but with marble from a different quarry.
Over time and weather, the two parts of the monument have become noticeably different colors, and a third type of marble in-between the main layers marks the delineation.
There's a "whispering gallery" in the US Capitol.
Visitors to the Capitol's Statuary Hall can experience the acoustical effect where something said at certain points yards away can be heard easily. These current points in the hall are different from those in the 19th century because the floor and ceiling of the hall have since been changed.
The National Cathedral holds a 3-billion-year-old piece of space.
The cathedral's Space Window took inspiration from photographs taken during the Apollo 11 mission and is inscribed with a Bible verse that says: "Is not God in the height of Heaven?"
The moon rock at the center of the window measures just over two inches, has been estimated to be approximately 3.6 billion years old, and contains the previously unknown mineral pyroxferroite.
Apollo 11 astronauts Neil Armstrong, Buzz Aldrin, and Michael Collins presented it to the cathedral in 1974 to mark the fifth anniversary of their mission to the moon.
The Maine Avenue Fish Market is the oldest in the country.
The market has been in continuous operation since 1805, making it the oldest open-air fish market in the United States. It beat the second-oldest, New York City's Fulton Fish Market, by 17 years.
There's a crypt in the basement of the Capitol that was meant for George Washington, but the first president never laid in rest there.
The underground space was completed in 1827 and not only marks the center of the building, but also the point from which the streets in Washington are laid out and numbered.
Though the crypt doesn't contain the first president, 13 statues are on display from the National Statuary Hall Collection to represent the 13 original colonies.
There was a typo in the original etching on the Lincoln Memorial.
A three-panel etching of Lincoln's second inaugural address on the north wall contained the word "future" spelled with an e instead of an f. The mistake has since been touched up, but the letter's extra leg is still visible.
Georgetown University's campus sits atop a network of underground tunnels.
The maintenance tunnels are popular among adventurous students who have been reported to visit the tunnels for graffiti, first-hand reports, and secret society meetings.
The White House has a near-twin in Dublin, Ireland.
It's widely believed that Irish architect James Hoban won a newspaper contest to design the White House based on a design inspired by Leinster House, which houses the Irish Parliament in Dublin.