A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

The remains of an abandoned dinosaur theme park litter the woods just off the highway.

Natural time capsule containing an alien ecosystem.

The 10-foot-wide Long Beach home was built on a dare.

The ruins of an 18th-century mansion that was once named "Mistake" by the man who had it built.

A historic bridge no one is allowed to cross.

In the mountains of West Virginia is this perfectly preserved Art Deco train station.

Mosques carved from coral and dozens of well-preserved stone structures fill this 500-year-old Swahili trading center.

In 1329, Christine Carpenter entered this small stone cell in the hopes of becoming a living saint.

The "World's Largest Underground Business Complex," a 55,000,000-square-foot city underneath Missouri.

Egyptian Revival movie palace abandoned for decades and restored in 2013.

Located in the world's highest-altitude Dark Sky Community, this observatory welcomes amateur and professional astronomers alike.

It's safe—and permitted—to walk across these twin natural sandstone arches.

The final resting place of one of the United States's greatest poets.

In the middle of a New England forest sit the crumbling ruins of World War II-era ammunition storage bunkers.

Once home to a thriving oil industry, this town now sits eerily abandoned.

An enormous, inside-out glass globe built in 1935.

Colin Stokes "got a bit carried away" and built a castle that looks like something out of Tolkien.

The camera-toting gargoyle juts out from the exterior of a Gothic cathedral.

Part of a highway network meant to connect Miami to Montreal, this 10-mile stretch of road made from red bricks is still around 100 years later.

The now-abandoned tunnels were built for the transcontinental railroad on the route where the first wagon train entered California.

An idyllic mineral hot springs facility with a more-than checkered past.

The ghost town that was one of New Hampshire's first settlements now exists as a park detailing its history.

The only surviving prototype for Buckminster Fuller's house of the future.