66,000 pieces of art, donated by one of the world's richest men.

The oldest surviving synagogue in New York City is now a soaring art space.

A living archive of urban activism and space reclaimed.

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

A hip bar decorated with walls of golden skulls.

Cryptic messages embedded in the New York City sidewalk.

New York's oldest magic store.

A graffiti Mecca welcoming artists from around the world.

Two New York rooftops are home to the largest rooftop soil farms in the world.

Townhouse facade doubles as an RMS Titanic Memorial.

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

A tunnel once abandoned now thrives with graffiti art as an off-limits makeshift gallery.

Enormous lake at the bottom of a unique cave system.

A lovers' beach, tucked below the surface of the island, provides a safe haven for romance.

An emerald-green grotto just a short trip from Austin.

Site of a former hidden treehouse in New Orleans.

Abandoned bridge hidden deep in the San Gabriel Mountains.

Bowl in splendor, if you're lucky.

A hyper-luxurious, 12,000- square foot home might be buried beneath Queens, still waiting to protect you from nuclear missiles.

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

In the middle of downtown Brooklyn lies a 165 year old secret hidden right below the surface (closed to the public indefinitely).

Elevated freight railway turned wildly successful urban park.

This beautiful building in the heart of Manhattan's Financial District was empty for decades.

La Monte Young and Marian Zazeela's "Dream House" will immerse you in an ever changing world of sound and light.