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

This historic hotel invented the chocolate brownie.

Once connected to the outside world by a single train track, this ghost town is looked after by the National Park Service.

A beautiful geyser under environmental threat.

Famously misunderstood wonder of the ancient world

Handsomely crafted representations of our round planet.

Only known mainland rookery of the threatened Stellar sea lion.

This museum devoted to the gothic author holds such interesting ephemera as his socks and walking stick.

A massive underwater sinkhole made famous by Jacques Cousteau.

A winding path built in secret on the edge of the continent.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

In the middle of an extremely unforgiving and desolate high desert is the most secret military facility in the world.

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

A tunnel once abandoned now thrives with graffiti art as an off-limits makeshift gallery.

The gorgeous library at Trinity College is home to remarkable treasures of Irish identity.

An underground city of salt.

The remains of an idyllic small town built for the film 'Big Fish' still stand around a rundown main street.

A crumbling staircase in upper Manhattan is all that remains of one of the most important sites in sports history.

Final resting place of two dozen New York City harbor ships.

Hidden in Central Park's Ramble is a troublesome cave buried just beneath the surface.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

A room in New York City that contains 250 cubic yards of dirt worth a million dollars.

The oldest living organism in New York City. Maybe.

A uniquely lettered street sign marks the corner where the iconic word game was invented.