About 35 million years ago—the dinosaurs are dead, but the Appalachian Mountains are still covered in tropical rain forests—a rock from space that was more than a mile wide and moving at supersonic speed crashed into the Atlantic Ocean off North America.
Supersonic? Now there's an understatement. We're almost certainly talking about an impact speed in the tens of miles per second compared to around 1,100 feet per second for the speed of sound.
Presumably what the writer meant by 'supersonic' was "really, really fast..."
The quoted "70,000 miles per hour" works out to roughly Mach 100.