This railroad tunnel was once the longest in Massachusetts.

Once the largest gravity dam in the world.

At the time of its completion, it was the tallest dam in the world.

Devilish landscape, once cinema home to giant alien bugs, now derelict roadside attraction.

The remote outlaw hideout of Jesse James, Butch Cassidy, and the Sundance Kid.

The world's largest mineral hot springs is home to a herd of bison, a free public bath house, beautiful mineral formations, and a terrifying swinging bridge.

The flow from this waterfall sometimes comes out of springs below its rim.

Each winter this underground cave forms a dazzling ice palace.

The largest known open fault in the United States is basically a giant natural refrigerator.

Chilly air billows out from the entrance of an abandoned mine on a steep hillside, providing a refuge for Ice Age fauna and flora.

This Native American petroglyph marks the location of an important, Creek trail.

A prehistoric rock shelter that's literally in someone's backyard.

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

A Buddhist monastery tucked away in the mountains of Colorado.

One of the largest Buddhist communities in the West sits on the site of an old hospital.

Naturally carbonated water fills the mineral baths at this 19th-century California resort.

A working model of a pressure-driven, high-speed rail whizzes around a private vineyard.

One of only five observatories in the world built to measure the Earth's wobble.

This extremely remote border crossing was closed to prevent smuggling and looting.

This lesser-known geographic marker was the reference point for all surveys of the continent for 75 years.

See the home and the range on which "Home on the Range" was arranged.

Small plaque marks the spot, but the true center is located about half a mile away on a private pig farm.

Over 600 acres of idyllic open grassland honor Nebraska’s georgic storyteller.

A museum dedicated to the quirky and colorful world created by Springfield's favorite son.