The abandoned remains of this once-thriving hotel looms over the city, awaiting an uncertain future.

Outsider Art Community.

The largest natural travertine bridge in the world is nestled in a small canyon in central Arizona.

A partial tunnel blasted into a steep ridge is all that remains of a failed railway across Arizona.

The eerie ruins of an isolated abandoned town nicknamed "the Widowmaker" for its deadly gold mine.

At this deceptively beautiful beach, nothing is what it seems.

Manhattan's Thirteenth Avenue was made on landfill then deliberately destroyed — apart from one small, obscure block.

These ornate manhole covers were used for a now-forgotten purpose.

The oldest operating apothecary in the US has treated everyone from Thomas Edison to Mark Twain.

New York City's smallest piece of private property.

The singular ruins of a mysterious lost city.

An organ located deep within a cave, whose "pipes" are the geological features of the cave itself.

The largest self-built castle in the U.S., six decades in the making.

An earthwork sculpture built by Robert Smithson in the Great Salt Lake only reemerges during drought.

The dairy farm in upstate New York where nearly half a million people gathered for three days of peace and music in 1969.

This collection of neon icons is preserving Arizona's history, one sign at a time.

This unique structure in suburban Austin is made up entirely of other people's castaways, and constantly changing.

The rustic cabin-like decor of this Arts and Crafts-style church reflects the love of nature central to its little-known theology.

A modern mechanical planetary system, part of a 10,000 year clock.

A huge musical instrument played by the ocean.

Death road of Bolivia.

A monument to the art of slicing people open.

An Aztec temple, the largest man-made pyramid in the world, sits buried in earth with a Spanish church set on top.

These strikingly colorful paintings were created by a mysterious Mesoamerican civilization.