A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

The country's oldest continually-operated mini golf course lies hidden in plain sight, eclipsed by one of D.C.'s most popular tourist attractions.

A Civil War fort in the middle of Washington, D.C. has been swallowed by a forest.

A wondrous old library overlooking the nation's capital.

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

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

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

A monument in Washington D.C. marks the spot from which all other roads were supposed to stem.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

The United States Capitol's former columns still stand.

This grave marks the passing of a historic Jane Doe whose identity remains a mystery to this day.

Parking garage where Bob Woodward met Watergate source "Deep Throat."

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

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

The ruins of this long-abandoned unsuccessful gold mine can still be found.

This pre-Revolutionary War grist mill ground corn and flour for soldiers in seven American wars.

A double agent hid information for the Soviet Union beneath this bridge for more than 20 years.

Exact replica of the ancient monument, but much, much lighter.