In the far northern reaches of the Siberian tundra is an enigmatic place called Popigai. The high cliffs along the rivers there are made of rock that shows signs of once having been completely melted, and satellite images reveal that the tundra actually forms a giant ring-shaped depression 60 miles across—which suggests that Popigai is a vast meteorite crater. Last July a team of Canadian and Russian scientists announced that they had determined when the meteorite hit: 35.7 million years ago, give or take 200,000 years. They calculated that date from the amount of radioactive argon that had decayed in the rocks since they resolidified after the impact. Remarkably, in 1995 other researchers had pinned the age of a 50-mile-wide crater now buried in the Chesapeake Bay to almost exactly the same time.
These two impacts—the two biggest in the past 65 million years, and among the biggest of all ...
