Around 200 sites where asteroids and comets have collided with the Earth are known on land and beneath the oceans. These impact scars vary in diameter from a few tens of metres to more than 100 km across, and range in age from a few thousands of years to more than 2 billion years old. Most impact sites are now just ghostly circular remnants in ancient eroded rocks, whilst a few are clearly visible as large, deep craters. This surviving record is the hard evidence against which predictions of the environmental effects of potentially catastrophic impact are compared. Estimates of...
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