The land is home to swimming ponies and a legendary 18th-century treasure.

An astounding collection of medical specimens, including bits of President Lincoln.

The Pennsylvania landscape depicted by a quarter mile of mosaics made from used road signs.

This Pennsylvania cave holds evidence of the oldest known inhabitants of North America.

A macabre farm reserved part of its 222-acres for dead bodies, all in the name of science.

Frank Lloyd Wright's most iconic home dangles over a Pennsylvania waterfall.

An abandoned stretch of highway, deteriorating next to the operational one.

Arlington Cemetery exhibit dedicated to death and grieving.

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

America's most famous museum of medical oddities is home to the remains of Albert Einstein's brain.

The feral felines of Roosevelt Island have a small army of allies and an abandoned hospital all to themselves.

A treasure trove of artifacts, books, and artwork from the "golden age" of exploration.

Tucked in the back of this amazing antique store is an important piece of medical history.

This beautiful building in the heart of Manhattan's Financial District was empty for decades.

A crumbling hospital from the 1850s on Roosevelt Island.

A terrific purveyor of natural history objects and curios.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.