Trails through lava-carved tube-caves display sparkling bacteria and a rich history.

A volcanic fissure over two miles long and up to 70 feet deep.

The victims of a free-floating Japanese bombing during WWII are remembered by this stone monument.

A roaring, awe-inspiring double cascade of icy water flows through woodland Oregon like something out of a Tolkien novel.

An interesting decoration on this historic site, this weathervane comes with as many legends as it does questions.

Berthed at the Charlestown Navy Yard in Boston, "Old Ironsides" is the oldest commissioned warship still afloat.

Macabre headstones carved with winged skulls, dancing skeletons, and pithy reminders of impending death.

The second-largest ceremonial mound in the United States is an artificial hill that is loosely shaped like a pentagon.

An eerie and beautiful group of columns mark the site of a grand mansion lost to time and fire.

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

An experimental, automotive replica of Stonehenge.

The Old Courthouse takes visitors on a captivating trip through St. Louis' timeline.

Guns that have been defending San Francisco since 1904.

The key to turning shifting dunes into the green oasis of Golden Gate Park.

Golden Gate Park's historic collection of plants.

An operating lighthouse from the 1860s.

A young botanist's horticultural vision realized through a lifetime of planting, tending, and collecting seeds.

A dramatic, 360-degree recreation of Pickett's Charge in the Civil War.

The United States’s oldest funicular railway glides up and down a steep Pittsburgh street.

Birthplace of the "Cat in the Cradle" and a cluster of iconic, original windmills that let the Dutch tame the seas.

The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

An immersive environment that is part haunted house, part choose-your-own-adventure, and part jungle gym.

A burnt-out motel with priceless Native American artwork in the basement, redeveloped into apartments.

This statue of an obscure comic strip hero stands as a tribute to patriotic service and a cautionary tale about embracing fads.