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

A mysterious castle built as a monument to lost love.

Dedicated to an "Unknown Man," this grave holds either the remains of a lovelorn suicide or a Cold War spy.

A gruesome and mysterious ancient sculpture of a pagan deity from the Roman occupation of Celtic Britain.

This cave system allegedly holds the "metallic library" of a lost ancient civilization.

The footprint of a frustrated devil in Munich's Cathedral.

This mysteriously shaped boulder may have been used for human sacrifice in ancient times.

A three-story copper skybridge connects two Manhattan buildings with Art Deco luxury.

Hidden in Central Park's Ramble is a troublesome cave buried just beneath the surface.

This cemetery has graves dating back to the 17th century, including the city's oldest carved tombstone and an ominous cryptogram.

A hands-on traveling collection of the innovative devices of yesteryear.

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

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.

Read a book next to a Midtown waterfall.

The feral felines of Roosevelt Island have a small army of allies and an abandoned hospital all to themselves.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

Manhattan’s Battery Park has a whimsical carousel full of spinning, glowing fish.

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

A crumbling hospital from the 1850s on Roosevelt Island.

A terrific purveyor of natural history objects and curios.

This giant boulder is sandwiched between apartment complexes in upper Manhattan.

A towering cemetery monument honors a man who threatened to expose the Freemasons and paid with his life.

This (occupied) house is 10 feet wide and full of spite.