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.

An extraordinarily ornate museum in San Diego holds wondrous anthropological treasures.

A small sculpture garden that provides a microcosmic representation of California.

A Biblically-based Natural History Museum.

World's first "penitentiary," meant to be humane, drove men insane.

Unrepaired walls from a 1920 anarchist bomb attack.

An old town theater converted into a celebration of the hobo legacy.