Death from Above: Mass Extinctions Tied to Comet Strikes

Many of Earth's mass extinctions over the eons have been caused by comet strikes, a new study suggests. Over the past 260 million years, cratering rates on Earth have peaked every 26 million years or so, in tune with a previously noted cycle of mass-extinction events, researchers found. Furthermore, five of the six largest impact craters known from the last quarter-billion years — including the 112-mile-wide (180 kilometers) crater associated with the demise of the dinosaurs 65 million yea