This moving cemetery sphere is a slowly rolling stone.

Legend says that this short Canadian tunnel still holds the dying screams of a girl who burned alive.

A well-preserved network of cavernous tunnels have spent silent centuries removing Toronto's waste.

A piece of crumbling highway infrastructure was repurposed as a high-tech facility for testing racecars.

This nondescript building hides an entrance to an underground nuclear tunnel system.

Washington's top-secret Cold War-era doomsday communications tower is located in a small neighborhood park.

These abandoned Cold War-era bunkers are now a designated refuge for endangered bat species.

A museum dedicated to the legacy of Bob Moog and his pioneering synthesizers.

These colossal desert barcodes have been fine-tuning spy satellites for decades.

A lone historical marker off the highway and scattered debris are all that remain of this Cold War-era missile site.

This sprawling farm was once a Cold War-era hideaway for the nation's top diplomats.

The world's first aircraft factory has been hiding inside an automobile plant for almost 80 years.

The enormous 250-room Vanderbilt mansion conceals hidden doors and secret passageways.

This medieval church is the only one in the world that features all Marc Chagall stained-glass windows.

A famed artist's inspiring work of peace and tolerance.

Barça football fans might see the insignia hidden in a church window as a divine sign.

A very classy 1920s-era bar right in Grand Central Station.

The famous facade of Grand Central Terminal hides a regulation tennis court—and it's not the first one.

A semi-scholarly approach to sex.

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

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

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

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."

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