A 140-year-old castle with underground caves hidden in San Francisco.

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

An artsy hidden staircase leads to breathtaking views of San Francisco.

A huge musical instrument played by the ocean.

A dark patch of the ceiling at Grand Central Terminal which was not restored is still stained brown by tobacco.

Read a book next to a Midtown waterfall.

The remnants of two World's Fairs are here, complete with a 12-story globe, a mini-Manhattan, and a UFO-shaped pavilion.

One of only two commuter aerial tramways in the United States.

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

The "Seinfeld" diner.

Visit the monolithic tomb of America's most famous general.

The first "Hall of Fame" in America was designed by legendary Gilded Age architect Stanford White for what was then the NYU uptown campus.

The one-stop shop for all of your superhero (or evil arch-nemesis) needs.

You can still ride this 100-year-old work of art in Brooklyn's largest public park.

This city park was once a lookout point for George Washington’s army during the Battle of Brooklyn in 1776.

New York City's smallest piece of private property.

The oldest living tree in Manhattan comes shrouded in sinister legend.

A dilapidated dock tells the story of Titanic's missed arrival.

A strangely angled West Village home is the only monument to an explosion that took the lives of three American revolutionaries.

This unmarked and unloved Manhattan subway grate created one of the most iconic images in American cinema.

A celebration of capitalism in Midtown.

The notorious headquarters of a corrupt political machine.

The rough and tumble president's childhood home displays the shirt he was once shot in and the speech that saved him.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.