The walls of the iconic dome are hollow and have a secret stairway.

A century-old network of freight tunnels underneath the city's historic business district.

The grand marble structure next to the White House is Andrew Carnegie's temple to Pan-American diplomacy.

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

D.C.'s first central library was born out of a chance encounter with the philanthropist whose name it bears.

A nostalgic bibliographic gem.

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

A wondrous old library overlooking the nation's capital.

Where the U.S. government kept its actual treasure, before Fort Knox.

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

This 16-foot, intricately detailed model of the Gearing-class destroyer has been called the most detailed model ship ever built.

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

One of just a few cities in the world to base their center of government on an island. Others: Paris and Osaka.

Seemingly unremarkable items like empty sushi trays, chef hats, and freshness stickers are being preserved so future generations can look back on this beloved cultural import.

This seemingly out-of-place gas station by the Watergate hotel was once described as the most expensive gas station in the world.

The most accurate timepiece in the world.

An atomic bomb-proof strongbox protects the U.S. Constitution from terrorists and thieves.

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

Once at the center of U.S. finance, the historic Trading Room was meticulously recreated piece by piece and rebuilt in the Art Institute.

One critic described it as "middle Mussolini, early Ramses, and late Neiman-Marcus." Another called it an architectural "natural disaster."

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

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

These chunky stone towers out in Lake Michigan are the key to Chicago’s drinking water.

"Congress may voluntarily remain in session throughout the summer, in order that our Congressmen may be protected from the intolerable discomforts and dangers of the ordinary outdoor weather!”