The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

The largest earthwork effigy in the world.

This ancient settlement contains some of the largest earthen mounds in North America.

This hunter-gatherer architecture is believed to be the oldest religious complex known.

These broken shards of ancient ruin may hide an ancient pyramid.

The remains of what was, not so long ago, the world's largest city.

A marvel of technology a century ago, this disused power station now offers a lovely glimpse into the industrial design of the past.

A battle between nature and architecture in the Cambodian jungle.

A massive underwater sinkhole made famous by Jacques Cousteau.

Every equinox this Maya pyramid puts on a spooky ancient light show.

Underwater art project will double as a massive artificial coral reef to help regenerate the damaged ecosystem.

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.

An emerald-green grotto just a short trip from Austin.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

The rocks in this arid basin leave trails of movement behind them.

"Only the devil could play golf here."

The ghost town that ended up on Ebay.

Factory that once supported an entire company town of 350.

The ghostly remains of an accidental sea.

Decommissioned experimental solar facilities that pioneered solar energy technology.

A ring of creosote bushes that are estimated to be 11,700 years old.

The "3rd largest" city in California is an empty mirage of suburbia in the middle of nowhere.

Abandoned hotel with adjacent lake and golf course.

Rusty industrial city full of oxidized distillation towers.