The oldest surviving bridge in New York City, which reopened to pedestrians in 2015.

This cemetery has graves dating back to the 17th century, including the city's oldest carved tombstone and an ominous cryptogram.

Try not to get slimed in Tribeca.

A crumbling hospital from the 1850s on Roosevelt Island.

Bar full of oddities, specimens, artifacts and homages to the great dime museums of the past.

This unassuming passageway played a key role in one of the most important events in U.S. history.

Also known as the Founding Church of Scientology.

Disks are rolled out onto the south lawn to absorb the impact of Marine One's wheels like giant coffee coasters.

D.C. once had its own tiny rival to the New York Stock Exchange.

This restaurant has been a haven for D.C.'s LGBTQ community since the 1950s.

This stately mansion, built in 1787 by America's first Secretary of the Navy, is rumored to be one of the most haunted buildings in Washington, DC.

A derelict bit of infrastructure from the canal that once ran through D.C. is landlocked in the heart of the city.

Was there a small "backpack nuke" hidden in the attic? JFK apparently thought so.

Behind an abandoned storefront is an example of pioneering 1930s gas station architecture.

Daredevil repair workers can worm their way out the access hatch, loop ropes over the apex and rappel down the monument.

A tiny monument to the unsuccessful attempt by Thomas Jefferson to place the prime meridian in Washington.

Helen Keller once broke ground on this historic center for the study of technologies to benefit the hearing impaired.

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

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

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

This unacceptably tall building was the real reason for Washington, D.C.'s skyscraper ban.

There’s a special club house at the top for the bell ringers.

Art deco landmark on the outskirts of Washington, D.C.

One of the largest artillery pieces in the world sits in a Washington D.C. parking lot.