Inky sludge oozes to the surface of this beach thanks to a rare natural tar seep.

A scenic fort on the harbor protected Philadelphia from an attack that never came.

This state park was once an important Native American cermonial center.

The remnants of a failed socialist commune can still be found just off a California highway.

Massive prehistoric mammals used these stones as oversized scratching posts.

The cave where two British judges hid in exile after sentencing the king to death.

The first artificial white water river course in North America cuts right through an Indiana city.

This lush Florida island is home to the ruins of a crooked ship breaker's tiny empire.

In memory of the 110 victims of ValuJet Flight 592 that crashed in the Everglades.

Once the world's largest reinforced concrete structure, this abandoned bridge now beckons urban explorers.

This 140-year-old beacon off the Florida Keys marks the wreck of a Navy ship that once roamed the Caribbean hunting pirates.

A New England-style lighthouse along California's beautiful Highway 1 gets a re-do.

A precariously balanced rock perches on the edge of a cliff created tens of thousands of years ago by glacial erosion.

This mountain tram provides stunning views while hanging from a single, steel thread.

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

An oddly poignant roadside attraction celebrating American folklore.

An abandoned network of military bunkers and tunnels open for anyone willing to descend into the darkness.

A century after his death, this covered bridge still calls to mind the plein air painter who loved it.

This stretch of beach along California's central coast is a busy highway for the largest seal in the northern hemisphere.

A double agent hid information for the Soviet Union beneath this bridge for more than 20 years.

A peaceful international sculpture garden in upstate New York.

In the middle of a suburban Denver neighborhood is a cave that once provided solace for a prominent Ute chief and his family.

This WWII-era military installation once held an entire city within its walls - now, its walls are falling down.

Crumbling remnants of what was once a booming town full of silver.