The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

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

One of the oldest used bookstores in the U.S. has been selling antiquarian treasures since 1825.

The charmingly disheveled clock tower of Tbilisi's puppet theater.

Deep in a Polish forest, this collection of Iron Age megaliths still holds onto many of its ancient secrets.

England's oldest surviving hedge maze has been confusing visitors for over 300 years.

This unique exhibit "Dialogue in the Dark" reverses roles as the visionless become guides and the sighted become ‘blind.’

Thousands of exquisitely carved stone water storage wells, hundreds of stone steps fill this exquisite medieval stepwell.

An optical illusion found amid a logged redwood forest.

The oldest stone house in Paris was built by its most famous alchemist.

This Polish cave has housed a dragon, a brothel, and an enduring legend.

A 20th-century picture show in 3-D.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.