An 18th-century Spanish watchtower made entirely of seashell and concrete.

The walls of America's oldest masonry fort famously "swallowed" cannonballs.

A gothic palace of endless stalagmites and pseudoscorpions waits within one of the world's most beautiful caves.

Lady Liberty's first beacon of enlightenment is now displayed in a museum near the statue.

A historic and oft-filmed prison stands in a Chicago suburb.

The fifth-longest suspension bridge the world, connecting one Michigan peninsula to another.

This family driveway also serves as the highest point in Illinois.

This high point in the Great Smoky Mountains is said to be home to the chief of all bears.

A large stone map of Arkansas awaits visitors to its highest point.

This trio of Australian rock spires may have inspired aboriginal legend or just modern tourism.

The prototype for all Nazi concentration camps.

Elvis's own personal tropical hideaway.

This marker identifies the location where three states converge.

This highest peak in New Mexico is named after an explorer who had a habit of lending his moniker to peaks.

A dangerous narrow-gauge railroad once traced a route through this striking canyon.

See massive fossils that are over 200 million years old in northeastern Arizona.

This 4,000-foot-diameter hole is touted as "the most well known, best preserved meteorite crater on Earth."

The largest collection of historic military firearms in the United States.

Considered by many to be one of the most dangerous highways in Alaska.

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

The longest living non-clonal organisms on Earth.

The largest island on the great salt lake is home to curious creatures and strange stories.

Famously misunderstood wonder of the ancient world

The first island in the world designated a light-pollution-free astronomical observation point.