Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.

The Cold War-era "underground Pentagon" is a sprawling subterranean hideaway for the Defense Department.

The complex has been abandoned since the Croatian War of Independence, barring a stint as a set for 'Game of Thrones.'

Once in disrepair, this popular Texas stop along iconic Route 66 has been restored to its past glory.

A pride of the state, this working museum crafts pottery mosaics in the style of Pennsylvania's earliest European settlers.

The stunning interior of this Gothic church is covered in ornate gold details.

The sounds of the world's largest fully functioning pipe organ dazzle shoppers at a Philadelphia Macy's.

A small museum dedicated to the whimsical tool that turns sugar cookies into works of art.

A tribute to an iconic act of defiance from the Croatian War of Independence involving a small car.

The abandoned "Ghost Mansion" was left to decay in the mountains of Northern Italy.

The final remnant of the Arkansas zinc industry is this historically protected ghost town.

Every hour the doors open to reenact the legend of the heroic mayor who saved his town by drinking a gallon of wine.

Three markers installed over the course of 140 years mark the point where Arkansas, Missouri, and Oklahoma converge.

Color-coded signs direct drivers on a network of routes designed to avoid Pittsburgh traffic.

This Croatian computer club features such gaming relics as the infamous Virtual Boy and an 8-bit Pacman machine.

Few other hotels can rival the grandeur of the Castle in the Rockies.

This truly palatial work of Communist-era architecture now sits crumbling on a Croatian beach.

It's estimated that half of the world's gold was mined from South Africa's "golden arc."

This spot memorializes one of the greatest plays in NFL history, unless you happen to be a Raiders fan.

A portion of the old Pirates stadium over which a defining home run once flew still stands even after the rest of the stadium was torn down.

Sections of the iconic limestone columns have been left uncleaned as a reminder of the city's polluted past.

This futuristic airport terminal was abandoned for decades before reopening as a hotel—with a vintage plane of its own parked out back.

This spectacular public market has got meat, fish, vegetables, baked goods, and enough kielbasa for a lifetime.

A stone arch leads to a cavernous 1930s spring house tucked away in the Pennsylvania woods.