These beautiful ruins deep in the Pennsylvania woods are thought to have been a ski lodge or a speakeasy, but no one really knows.

One of the most unusual geological features, recognizable from a dozen hit movies.

A sculpture for the sun and stars, created by Nancy Holt.

Long the exclusive domain of birds and vagabonds, this little-known Central Park peninsula is open to the public once more.

One man has installed bathtubs into the Utah desert floor to harness the powers of a natural hot spring.

An overlooked and unassuming engineering marvel.

Mile-long cave system near Great Smoky Mountains.

A grove of colossal man-made flowers built to collect the sun's rays.

The first park built over a freeway is a brutalist masterwork.

Abandoned silver mine strewn with wrecked and twisted structures.

A park full of outsider art, created by college students studying outsider art.

These sub-farmland caves are home to a majestic stalactite known as the "Crystal King."

The now-abandoned tunnels were built for the transcontinental railroad on the route where the first wagon train entered California.

This large midwestern cavern features impressive rock formations and an underground waterfall.

This abandoned subway station can still be seen on regular commuter trips.

Giant pinnacles in the sun-scorched lands of Southern Utah have been nicknamed "white ghosts."

San Franciscans leave gifts and messages for the magical beings that live behind the tiny, mysterious door installed on a tree in Golden Gate Park.

Japanese gardens dedicated to the Japanese farmer that refused to throw in the trowel.

Rare recess caves fill this unbelievably beautiful Ohio park.

These Great Lakes sea caves accessible by boat in the summer turn into temples of ice reachable by foot in the winter.

The only remnant of a small mining town is an abandoned railroad tunnel that is steeped in ghostly lore.

Bad farming practices caused this unnatural natural wonder.

The often-overlooked ruins of one of the first buildings designed by famous landscape architect Frederick Olmsted.

These Pre-Columbian petroglyphs contain representations of birds, deer, hunters, and even some early Native flute players.