A giant, Gilded Age mansion that once belonged to the beer baron of Milwaukee.

Vikings and Evangelicals have both had a hand in this replica medieval church.

The marker offers a rare chance to be in four U.S. states at once, though there's some controversy over its geographic accuracy.

Once the largest windmill outside of Holland, with enormous 114 foot sails.

Guns that have been defending San Francisco since 1904.

The key to turning shifting dunes into the green oasis of Golden Gate Park.

Golden Gate Park's historic collection of plants.

The seawater playground of gilded-era San Francisco burned to the ground in 1966.

Relics of the 1894 Midwinter Exposition continue to delight visitors today.

Carnivorous plants, medieval aphrodisiacs, and the largest original wood structure glass conservatory in the western hemisphere.

On the Fourth of July this area will transform into a Pompeii-like storm of falling embers and ash.

The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

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

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

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

A building shaped like the Army Corps of Engineers logo, because why not?

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

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

Before Metro, Washington had a robust streetcar network—and you see the remains of this infrastructure if you know where to look.

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

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

The national park was once a plantation estate.

The "lovely red Vega" of the legendary record-settling pilot.

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