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

The most accurate timepiece in the world.

The only Civil War battle in Washington D.C. took place near this highest natural point in the city.

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

The Conduit Road Schoolhouse is one of the last one-room schoolhouses remaining in Washington, D.C.

A rare example of a rural school in Washington, D.C.

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

A monument marks the location of the first successful steamboat demonstration.

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

This mighty white oak in Bethesda survived both the American Revolution and the construction of Washington D.C.'s metro red line.

Surrounded by traditional homes, this Tolkien-esque dwelling stands out on a quiet street in suburban Maryland.

The last holdout of a pre-Civil War rebel who took the matter of slavery into his own hands.

This shale formation along the Appalachian Trail once inspired the third president of the United States.

Built in the 1850s, this church was of the first five churches constructed in Harper's Ferry.

This advertisement for talcum powder was painted on the rock face and has since defied removal.

FDR's Art Deco hospital tower was hopelessly inefficient.

This small hilltop church weaves the urban history of Washington, D.C. with the national history of the United States.

Once home to seven different roller coasters, Glen Echo has undergone many transformations since its founding in 1891.

When it comes to handsignals (and colonialism) rock always beats scissors.

Proto-CIA agents trained with grenades in the sand traps and fired machine guns from the fairways.

One of the oldest standing structures in Montgomery County, Maryland.

The bridge's highly-contested plaque (once again) honors Confederate President Jefferson Davis.

This massive swimming pool is the high tech home to the human-powered submarine races.

The oldest surviving one-room schoolhouse in Montgomery County, Maryland.