Remants of foundations, streets, and streetcar tracks inhabit the cliffs where a landslide occurred in 1929.

A 140-year-old castle with underground caves hidden in San Francisco.

These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

This secluded aqua waterfall in the Grand Canyon is the perfect swimming hole, and the Havasupai tribe's fiercely protected natural wonder.

Wondrous sandstone hoodoos where Native Americans collected clay for pottery, 9,000 years ago.

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

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

These old stone ruins in the Oregon wilderness were once bathrooms and are steeped in legends of murder.

An abandoned cemetery lies in ruins as the surrounding forest swallows it whole.

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

On the Oregon coast there is a natural hole that seems to be draining the sea.

Most-visited and most-photographed slot canyon in the American Southwest.

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

Rippling sand dunes frozen in the Arizona rock.

For a brief moment, the world was given a flicker of the future of fast food, thanks to a visionary inventor.

This notorious Canadian mental institution was home to some of the first LSD experiments and origin of the word "psychedelic."

What was once the largest mine in North America has left behind a hollow hill filled with bright blue waters.

Secret Swing.

A marvel of engineering built to prevent annual floods from devastating the city of Winnipeg.

Three young men who lived on this street are honored for their service during World War I.

This illuminated sign was the first of its kind in Canada and remains a vestige of the Atomic Age.

When North America was covered in ice, this was an oasis in the frozen desert.

This now abandoned arch bridge was constructed during the Great Depression.

Extremely long cable tunnel buried under the river.