A sculpture for the sun and stars, created by Nancy Holt.

The remains of a 19th century spring, and the site of an infamous unsolved murder.

Hidden in plain sight on the Hudson River is an abandoned train station where nature has taken over.

A bronze monument to the infamous radio broadcast in the real New Jersey town where the fictional Martians crash-landed.

In the power of this waterfall, Alexander Hamilton saw the potential for industry.

This whimsical group of statues is a favorite of children who love to climb all over Lewis Carroll's beloved characters.

A memorial to the first person killed by a motor vehicle in the Western Hemisphere.

Commemorating the spot where the famous scientist fed his pigeons.

On the Hudson resides one of the largest clocks in the world.

Spinning silently in the lobby of an Art Deco masterwork is a massive vintage model of the Earth.

Curious crustaceans support an obelisk from ancient Egypt that now stands in modern-day Central Park.

Amid skyscrapers and traffic exists this unique aquatic ecosystem.

This unassuming retro burger joint was billed as “diner of the future” at the 1939 World's Fair.

The "Key of Keys" for Washington, D.C. and the apocryphal landing place for the military campaign that started George Washington's career is now at the bottom of a well.

This nine-story building is the oldest standing skyscraper in D.C.

A replica of the famous memorial statue at the Normandy American Cemetery hides in an office building in downtown D.C.

Beneath unsuspecting visitors feet, a capsule containing pieces of America’s civil rights history.

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

A heroic pigeon that, through a barrage of gunfire, delivered a message that saved over 100 lives in World War I.

Press 1 for knock-knock jokes.

This bronze boar's snout has been rubbed to a golden sheen by visitors seeking good fortune.

This tiny cottage surrounded by railroads is the oldest surviving structure in Fairmount Park.

Contrary to popular belief, this secluded statue doesn't represent Pennsylvania's founder.

The oldest surviving presidential residence in the United States.