An uber-secure, windowless tower of doom in the center of Manhattan is an NSA spyscraper.

This memorial honors thousands of enslaved Africans and their descendants who died in colonial New York.

Deep under Manhattan is the largest construction project in city history. So far, the project has taken the lives of 23 workers, or "a man a mile."

Stay in a room with a view... of the bottom of the ocean.

Welcome to old Florida, where a 1940s mermaid show is still enchanting visitors.

Underground vaults of a former brewery beneath Brooklyn's Crown Heights are now used to ripen cheese.

A massive cave complex boasts some of the clearest water (and most charming bats) in the world.

These ornate manhole covers were used for a now-forgotten purpose.

Little Red Lighthouse under the George Washington Bridge.

Dine on food tied to Viking history and legend at this subterranean Stockholm spot.

Underneath the bustling city streets, a hydroponic farm grows unique herbs in an urban oasis.

More than 250 sakes are available in an izakaya hidden in the basement of a Midtown office building.

Approximately 300 million years ago, a massive coral reef existed in what is now the largest limestone cave in Japan.

A mysterious monument meant to be a guide into "an Age of Reason" was destroyed after an apparent bombing.

Just your average Marie Antoinette–themed speakeasy in an abandoned 19th-century subway tunnel.

This semi-spooky remnant of an abandoned hotel provides fantastic views.

Under a glass sidewalk lie the remains of some of Manhattan's oldest buildings.

A bizarre house filled with an astounding array of collections.

The "Bridge Over the Atlantic" lets you cross the waterway in mere seconds.

Overlooking Puget Sound sits a garden with a Buddhist prayer wheel and an elaborate labyrinth.

Hidden in one of New York's oldest subway stations is the final remnant of the site of the bizarre Shakespeare Riots.

Local legend says Isaac Newton built this footbridge without any screws, bolts, or nails.

A labyrinthine underworld of tunnels and caverns hides beneath the city.

Enormous lake at the bottom of a unique cave system.