An Industrial Revolution-era public work that purified water using nothing but sand.

These recently uncovered walls are all that's left of Washington, D.C's first defense contractor.

A historic ruin just 20 feet away from a busy highway onramp.

Once the supplier for noteworthy projects like the National Cathedral, this old brickworks now lies abandoned.

You could be standing at the site of one of D.C.'s most fatal tragedies and not even know it.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

A derelict bit of transportation infrastructure hidden in the woods.

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

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

This once-palatial plantation may have been the site where a draft of the Declaration of Independence was written.

Where Aleister Crowley taught magick in "La Chambre Des Cauchemars," his "Chamber of Nightmares."

At least seven hues of blue and turquoise make this freshwater lagoon unique.

These azure waters were thought to lead to the Mayan Underworld.

If you can look past the nearby sewer pipes, this urban sinkhole grotto is a mini-paradise.

A flooded Mexican cave that hides a river beneath the water.

An ancient system of freshwater caves filled with animal bones, including that of species long-extinct.

Unparalleled stargazing, one of Colombia's best observatories, and a swimming pool can all be found in the middle of this ancient tropical forest.

These ruins of a pre-Columbian cliff village are among the most beautiful and well preserved in New Mexico.

A small metropolis of Pueblo cave dwellings have been carved right into the hillside of this national monument.

A multi-storied adobe complex has been inhabited for more than a thousand years.

Twice a year, visitors can tour the desolate site that birthed the Atomic Age.

These aggressively sustainable art homes look like something out of 1970's science fiction.

A phenomenal assembly of pueblos in New Mexico is the most complete example of ancient ruins north of the border.