Certainly not a current environmental question, but a supremely interesting one nevertheless, is the age of the Grand Canyon and what shaped it.
Conventional thinking is that it was shaped by the Colorado River some 5-6 million years ago. But a new study led by researchers from the University of Colorado Boulder, published in Science, contends that the Grand Canyon is far, far older than that, and was carved by another river entirely, and running in the opposite direction to the water currently coursing through the canyon.
CU assistant professor Rebecca Flowers says, "Our research implies that the Grand Canyon was directly carved, to within a few hundred meters of its modern depth, by about 70 million years ago."
The conventional dating of the Grand Canyon is derived from the age of gravel washed downstream by the Colorado River. But the certainty of that 5-6 million years old date began to be eroded by the discovery of 17 million year old mineral deposits in caves lining the canyon walls. The even older dating suggested by Flowers is the result of looking at the radioactive decay of uranium and thorium into helium in the mineral group apatite.
If this latest study is correct, it means that the formation Grand Canyon predates the extinction of the dinosaurs by some 5 million years and before the modern Colorado River system. At that time, because of different topography, the ancient river that carved the Grand Canyon would've flowed in the opposite direction as does the Colorado today.
Keep in mind that the Grand Canyon is over a mile deep in places (1.6 kilometers).
So, if it was within a few hundred meters of current depth 70 million years ago, how much farther back in time do we have to go before we come to the start of the canyon formation, when a river had carved enough land away so it was no longer just a river with high banks and began the journey into becoming an ordinary canyon and then one of the seven natural wonders of the world?