The largest self-built castle in the U.S., six decades in the making.

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

An experimental, automotive replica of Stonehenge.

The closest thing to Mars on Earth.

A sacred lake composed of over 300 separate pools of highly concentrated minerals.

This massive underground salt mine now holds a sci-fi theme park in its depths.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.

Medicinal spa created with the discharge from a geothermal energy plant.

Hidden in the grounds of the Fort Dodge Museum is a "real" and terrifying petrified giant.

A unique attraction and museum dedicated to exploring iconic roadside structures.

Thousands of hats, pens, and pencils, hanging inside the walls of a former church.

An unusual geological feature towers over the remains of a dark episode in United States history.

This giant tower beacon was a key navigation tool for pilots delivering mail amid the night skies.

An apocalyptic seismic event creates a deadly chain reaction that ends with a 6-mile long lake developing within weeks.

Creek water that starts here has a 50/50 chance of ending up in the Atlantic or the Pacific.

The only natural lake in the world that drains into both the Atlantic and Pacific Oceans.

Cockatoo Island is a Heritage-listed island in the middle of Sydney Harbour.

The pillar of this bridge contains a small home and underneath, a ballroom.

Brisbane's manmade urban beach has enough water to fill five Olympic-sized swimming pools.

Its collection highlights the monumental tasks undertaken to map Queensland.

An exhibit of endangered species confiscated from smugglers and passengers.

This abandoned railroad tunnel is now home to a micro-colony of bats and a ghost train.

This natural sandstone bridge hasn't been seen since the 1960s when Lake Powell covered it—until now.