The Colorado River emerges as a tiny stream high in a rocky mountain meadow.

From that inauspicious beginning it rolls and roams across 1,500 miles of the western United States.

By the time it discharges its power in the Gulf of California, it has flowed through a world forever altered by its passing.

Chief among its creations is the mile-deep wound in the lime and sandstone of Arizona.

Over millennia it has carved down and down into the soft sedimentary rock,

sculpting the giant natural cleft we now call the Grand Canyon.

The Grand Canyon has long been a place of wonder. Inhabited by humans

for 12,000 years, it now draws 5 million tourists and adventurers each year who flock there

to hike its trails, clamber on its vast red walls, and revel in the almost cosmic scale of one of nature's most awesome sights.

And yet, only 5% of visitors ever leave the relative safety of the canyon rim.

Deeper down is where its true mysteries are found. Even its age is open to doubt.

Common geological wisdom dictates that the canyon was formed between 5 and 6 million years ago,

but the rock itself poses troubling questions.

You see, the canyon is missing time.

A lot of time.

Rock from the Paleozoic era sits atop a layer known as the Vishnu basement rocks

and the roughly 1.2 billion years of geological strata

that should separate them is simply not there.

It was Major John Wesley Powell, a soldier, geologist and explorer who in 1869 discovered this literal manifestation of missing time.

And in the century and a half since, no one has

been able to satisfactorily explain the absence.

It's a worldwide anomaly known as the great unconformity, but it is greater, more apparent,

...