The beautiful tunnel that ran from the lobby to Penn Station is still hidden underneath 34th Street.

The New York hotel where tragic visionary Nikola Tesla spent his final hours, destitute and alone but for the pigeons.

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

An extensive collection of fine art devoted to man's best friend.

A replica of a 7th-century church hides in plain sight in the middle of the concrete jungle.

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

The world's oldest hologram gallery and the labyrinthine laser laboratory of a holography pioneer.

Cryptic messages embedded in the New York City sidewalk.

A whimsical, but often overlooked clock in Park Avenue.

The one-time epicenter of American songwriting is now a little remembered Manhattan commercial block.

Unlock the secrets of the conjuring arts.

A semi-scholarly approach to sex.

A Manhattan courthouse hides a small but scathing memorial to Holocaust injustice.

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

Get a privileged view over Mexico City's Historic Center from a terrace right in its heart.

Gilded heaven for philatelists and architecture freaks, still in full working order despite sitting atop tremulous ground.

This covert museum showcases booties worn by a rescue dog and slippers owned by Queen Elizabeth II.

Discarded animal sculptures stand forgotten by an abandoned aquarium and water park in Bosque de Chapultepec.

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

A Volkswagen Beetle decorated with millions of beads connects Mexico's past and future.

Visiting this cacophonous collection of vintage novelties is like stepping into the mind of a crazed toymaker.

This "City of Books" holds a museum, library, and the complete personal book collections of five of Mexico's greatest thinkers.

This once-abandoned mansion is now a dreamy event space.

This warehouse-gallery-shop hybrid showcases Mexican craftwork at its best.