River pirates, murderous bandits, and serial killers used this Illinois cave as their debauched hideout.

Chicago's North Park Village was once the largest municipal sanatorium in the country.

A lush green oasis in the middle of Chicago.

Four glass boxes hover over 1,000 feet in the air to give visitors an unparalleled view of Chicago.

Natural rock water slides hidden in the remote forests of northern Idaho.

A legal loophole makes it possible to get away with murder within this 50-square-mile section of Yellowstone.

Jail from the 19th century that was built by inmates.

These black pillars of volcanic basalt seem to call mountain climbers with an unbreakable siren song.

Death comes to anyone who dares to touch this concrete column.

An abandoned railroad tunnel forgotten by time awaits inquisitive hikers.

"X" marks the spot of an echo chamber in the middle of a plaza.

The remains of one of America's first radiation testing grounds still rest ominously in a Georgia forest.

The perfectly named cave drop is the deepest in the continental United States, almost matching the height of Seattle’s Space Needle.

A curious whirlpool formation supposedly capable of entangling or engulfing branches, tree trunks, or whole rafts full of hapless tourists.

Constant erosion has created a surreal landscape of bleached and preserved fallen trees.

An underground magnet for ever-changing street art.

Ruins of a Carnegie family mansion on Cumberland Island.

One of the largest-ever mental hospitals now sits abandoned, surrounded by 25,000 unmarked graves.

Crumbling ruins mark the site of a couple's grisly murder in the woods.

The beauty of urban decay, hiding some dark secrets.

A mysterious monument meant to be a guide into "an Age of Reason" was destroyed after an apparent bombing.

The remnants of the base where US Army troops learned how to fight in extreme conditions.

A coal fire has been burning underground here since 1896.

Unlike most arches, this geological oddity formed in a volcanic dike.