"We know that the impact occurred within the last 70 million years, and in that time about 1.5 km of sediment has been eroded. That makes it really hard to pin down and actually date the impact." Erosion has worn away all but the "roots" of the crater, leaving a semicircular depression eight kilometres across with a central peak. Schmitt says that when it formed, the crater likely reached a depth of 1.6 to 2.4 km—the kind of impact his graduate student Wei Xie calculated would have had devastating consequences for life in the area. "An impact of this magnitude...