This nine-story building is the oldest standing skyscraper in D.C.

Every drop of D.C. tap water flows through this old waterworks.

The early data processor factory founded in Washington for the 1890 U.S. Census went on to become IBM.

A portal into the bricked up canal that runs through the heart of Washington D.C.

This robust fence in front of a historic Georgetown home is likely made from hundreds of recycled Revolutionary War firearms.

The discovery site of the "Capitalsaurus," the official dinosaur of Washington, D.C.

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

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

Obscure Freemasons still live in D.C.’s largest private residence.

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

They sort through millions of dollars worth of diplomatic gifts on behalf of the government.

Need a hardcopy of the 50-title Code of Federal Regulations? This is the place.

The national park was once a plantation estate.

This forgotten boat elevator was an engineering marvel in its heyday.

A bas relief commemorates the "Nuns of the Battlefield" who cared for soldiers on both sides of the conflict.

A fantastic array of trays and cables once whisked books over to the Capitol at 600 feet per minute.

Washington's top-secret Cold War-era doomsday communications tower is located in a small neighborhood park.

Washington's iconic carousel has a nice piece of Civil Rights history.

An overlooked oasis of quiet on the grounds of Washington's monumental basilica.

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

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

These recently uncovered walls are all that's left of Washington, D.C's first defense contractor.

100 years later, they're still down there.

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