This Arizona observatory is famous for investigations into Martian life, and as the place humans first spotted Pluto.

Deep scars on the earth left by the Ice Age.

A priceless collection of the Nation's cryptologic history.

These ruins of a pre-Columbian cliff village are among the most beautiful and well preserved in New Mexico.

Hordes of precariously balanced stone figures crowd the shores of the Hudson River.

Beyond the usual stories that attend old prisons, this one has deep bloody undercurrents.

The highest paved road in North America winds its way up Mount Blue Sky in Colorado to an altitude of 14,130 feet above sea level.

One day a year it is legal to jump off of this astoundingly tall bridge that spans an Appalachian gorge.

Once strictly off-limits, this lower level housing curious and priceless vehicles of the Petersen Automotive Museum is now open to the public.

This abandoned Civil War fortress is slowly being swallowed by the waters of Lake Borgne.

This super steep hike ascends 2,000 feet in less than a mile.

New York's oldest and possibly grandest keeper of books.

Limestone caves with spectacular showcase chambers.

A desert ghost town—occupied for only 12 years—was once home to more than 500 silver mines.

A 19th century library favored by Edgar Allan Poe and H.P. Lovecraft, known by locals as "the Ath."

Once a menacing challenge for trailblazers, this trail is now the most scenic drive in Utah.

A cave's rare beauty held a cavernous secret hidden underground.

This sacred cavern, 400 feet deep, is home to millions of bats and an ancient burial ground.

Years of archaeology, just chillin' by the water cooler.

This cave holds a surprisingly colorful variety of geological oddities.

Lie in a hammock where George Washington stood.

Peruse rare tomes like poetry bound in pearls or books with branches and berries growing out of their spines.

Thousands of artifacts relating to human sexuality, including the world's largest sex bike.

Over 50 whirling pools left behind from the glacial age.