An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

Home to items never before seen by the public.

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

The most confusing clock in New York.

Dodge prohibitionists and slide through the telephone booth in Crif Dogs.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

Elevated freight railway turned wildly successful urban park.

A Pythagorean funhouse that tries, successfully, to prove that math is the coolest thing ever.

New York City's smallest piece of private property.

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

More gold than anywhere else, ever.

Wall Street has a new heroine, a bronze statue of a small but fierce girl.

Hidden in plain sight on a pair of Manhattan subway tunnels are a pair of public instruments that sing to each other.

One of the last few places in Manhattan that employed a pneumatic systemic keeps it on display.

Tucked away amidst some of the most famous addresses in the world is New York's only fraction of a street.

The oldest operating apothecary in the US has treated everyone from Thomas Edison to Mark Twain.

Ride the subways of yesteryear.

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

This innocent-looking townhome is actually a subway exit.

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."

Neon signs that grace the windows of Bergdorf Goodman's and Tiffany & Co. come from this studio in Brooklyn.

Plot your trip through the city on the sidewalk.