Decades after testing the polio vaccine on unwitting patients, this historic mental hospital sits in ruin.

These World War I shooting range tunnels are now haunted halls of underground graffiti accessed through the roots of a tree.

This derelict building used to house infirm Freemasons, their widows, and orphans.

A towering cemetery monument honors a man who threatened to expose the Freemasons and paid with his life.

This simple stone memorial remembers a secret, Geneva Convention-defying interrogation base from WWII.

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.

The only National Monument ever to go missing for nearly 50 years then resurface in a dump.

Please don’t anchor your boat onto the Department of Defense’s underwater data cables.

The controversial urban planner who designed Washington, D.C., was buried in Maryland, and can presently be found in Virginia.

The naval commander’s proudest moment in battle is memorialized in this unique tomb at Arlington Cemetery.

A mammoth pair of Civil War artillery guns abandoned out in the forest.

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

This stately building in Alexandria, Virginia was built by Freemasons to honor one of their most famous members.

This imposing Masonic temple a mile from the White House was the first public library in Washington, D.C.

Huge spherical rocks in the middle of the prairie.

The atmospheric road was never meant to be so creepy.

The development and impact of electronic media showcased in a picturesque farmhouse in Maryland.

The Forest Service's surprisingly sophisticated collection of wildfire prevention art features America's most beloved bear.

A priceless collection of the Nation's cryptologic history.

Captain John Smith thought these cliffs were amazing in 1608 but sharks thought so 20 million years before him.

Rediscovered after a century, the father of the American Navy was reinterred in something dredged up from Davy Jones' Locker.

The elevator hidden in a smokestack on Roosevelt's yacht now serves as a monument to the former president.

A collection of outdoor art by renowned muralist Robert Dafford depicts the history of a flooded city.

There's a crumbling old mansion inside the Smithsonian National Zoo.