The way that light hits a stained glass window on a specific day is one of many "clues" to a mysterious treasure.

A planet-hunting astrograph built in the 1920s to seek out the so-called "Planet X" beyond Uranus.

A short but powerful waterfall resides amid Colorado’s Great Sand Dunes.

The precious-metals strikes dating back to the 1860s are now mostly inactive, but this area has become the United States' only domestic source of lithium.

This Gold Rush ghost town was once the subject of a territorial dispute between California and Nevada.

Two geoglyphs that are truly unique to North America.

Drive up a paved road into "Nevada's Yosemite" for exquisite views and mountain hiking in an alpine setting.

Built as a schoolhouse in 1914, this Mission-style building has also served as a dancehall, a library, an Army canteen, and now a cultural museum.

A mining town, now abandoned, was built in a canyon so narrow that the railroad ran right through the hotel lobby.

Many generations of travelers have passed through the steep, smooth walls of this canyon.

A breathtaking 19th-century trail overlooks Lake Tahoe.

A grove of "candelabra redwoods" deep in California's Lost Coast.

A faux desert landscape in the middle of the jungle that sprouts oases during the rainy season.

Low tide reveals 8,000-year-old rock carvings in Southern Alaska.

The ghostly remains of what was once the world's richest copper mine.

A porcelain company's former dumping ground, completely covered in pieces of collectible ceramics.

This subterranean oyster house has been the pearl of Grand Central Station since 1913.

Grotto replete with mystical designs laid out in glued shell mosaics.

Native American burial mounds? Giant gopher dens? No one knows what created the Mima Mounds.

One of the highest mountain passes in Europe has been ranked among the best driving roads in the world.

Bootleggers ran booze beneath this building before (and after) Route 66 appeared beside it.

A spring-fed pool so deep it's used for scuba diving as well as swimming.

The century-old remains of an abandoned farm town in eastern Washington.

A memorial to the beloved brown bear that served in the Polish army during World War II.