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The very largest of these comets are certain to leave their calling cards on the earth. One certainly was left at Tunguska in Siberia in 1908. What caused the giant explosion there has been a puzzle for some time. But I think it was a comet. It formed a shallow creator and felled trees for several hundred square miles around it. This is exactly what we can now expect to happen when a large water comet flies into the earth. The impact created only a shallow creator, which is not surprising since the impact pressures for water comets are 10 to 50 times less than for a solid rock. But the heat from the incoming hot piston of gas charred the area around the crater. And the atmospheric pressure wave it created left behind an eerie landscape of felled trees, all pointing away from the centre of the impact.<\/p>\n\n\n\n