A sacred site for Native Americans surveyed by George Washington and owned by both King George III and Thomas Jefferson.

Aqueducts have dramatically changed this old lake, now home to tufa towers and its very own species of tiny brine shrimp.

A massive volcanic plug is now a protected area that hosts a number of bird species.

One of the world’s largest and deepest freshwater springs, where mastodons once roamed.

At this Minnesota state park you can easily wade across the source of the mighty Mississippi.

The only Florida state park with publicly accessible caves.

Stroll through a wonderland of stupendous moss-covered trees.

The fifth-longest natural arch in the world, and the longest outside China.

The marker offers a rare chance to be in four U.S. states at once, though there's some controversy over its geographic accuracy.

This spectacular track has run continuously since 1882.

John Hendrix predicted the creation of the "Secret City" built as part of the Manhattan Project.

This former maximum-security prison is creepy, sobering, and supposedly haunted.

A bronze sculpture honors the 12 black students who attended the first integrated public high school in the segregated South.

Better known as the "Body Farm," this unique research facility has been studying how human bodies break down since the 1980s.

In one of America’s oldest mountain ranges, a long-awaited section of a scenic highway is now open to the public.

Mile-long cave system near Great Smoky Mountains.

This museum pays tribute to the Cherokee leader who created a written language for his people.

Abandoned for 60 years, this submerged bridge was completely forgotten until the lake was drained.

The ancient water tanks remain some of the ruined city’s most beautiful architecture.

The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

An ornate Art Deco tile mosaic representing Nebraskan life, both modern and prehistoric.

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

A tribute to Ponce de Leon's supposed quest for eternal youth.

An ivy-covered arch bridge dedicated to Tejano music singer and legend Rosita Fernandez.