A mysterious echo seems to defy the laws of acoustics.

The world's longest known cave system.

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

Where manatees are the snowbird vacationers in these balmy Florida springs.

Take a drunken tour inside the world's largest known geode.

Private collection of art inspired by cabinets of curiosity.

Each year these high desert hills are absolutely covered with poppy flowers in a surreal show of nature.

One of the world's oldest and most massive living organisms is a grove of quaking aspens.

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

A room in New York City that contains 250 cubic yards of dirt worth a million dollars.

This knobby, drooping New Orleans oak is a favorite place both for climbing and for meeting giraffes.

A huge musical sculpture and other art installations on the National Oceanic and Atmospheric Administration grounds.

The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

A breathtaking array of ancient Pueblo structures and artifacts counterintuitively preserved by neglect.

San Francisco's newly reopened hands-on science museum.

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

Collapsing castle on the Hudson river was once the storage site of military surplus business.

The hike through a narrow river flanked by the 2,000-foot-tall walls of Zion Canyon is one of the best in the world.

This frequently-photographed waterfall takes only a short hike to visit.

Two thousand artifacts from around the world collected by one woman who loved to travel.

Seemingly grown on some other world, these New Mexico rock formations look like a disused science fiction set.

A walk-in working camera obscura produces 360-degree live images of the San Francisco coast.

A stunning, two-tiered waterfall made famous by poets, artists and the legend of a ghost dog.

A favorite haunt of dinosaur loving children and hallucinogen loving hippies alike.