Largest Roman Catholic church in North America.

An obscure federal monument honors the French diplomat who served as Ambassador to the U.S. during WWI.

Politicians, bureaucrats, and military officers gathered in this field near the Capitol to settle their disputes with blood.

A clone of a cloned tree that was so important it allegedly had its own guards.

The semi-reclaimed ruins of this canal were once part of an ambitious plan to reroute the mighty potomac.

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

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

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

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

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

A memorial to a shareholder activist feared by CEOs around the United States.

Using a massive antenna made up of five miles of wire, two scientists tuned into organic radio waves from the planet Jupiter.

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

This suburban house is made of dirt, and almost sparked a revolution in home construction.

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

This ruined quarry once produced the distinctive red sandstone used for construction projects in Washington, D.C.

One of the oldest Catholic churches in the U.S., situated just east of Gettysburg.

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

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

An intricately designed junk art house with four extravagant junk art cars hidden in the suburbs.

Dating to 1793, this is the oldest building in Poolesville.

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.

A diminutive plaque recalls the treetop sniper who almost killed Abe Lincoln.