It is natural to ask why the moon should have suffered so many blows from space while the earth, probably its close companion for several thousand million years, seems to have escaped. As soon as scientists began to search seriously for impact craters on the earth, they found plenty of them. Maps and aerial photographs have led the searchers to many other intriguing circular formations. In the very old rocks exposed in Canada, for example, several large, almost perfectly circular lakes have been identified. Their features can be explained only as the result of meteorite impacts.
Similar structures exist in many other parts of the earth.
There is seldom any trace of the meteorite itself, but this is not surprising. Unless it hits at unusually low speed, a meteorite will vaporize and spurt out of the crater as gas. The largest structure on earth that is suspected of being part of an astrobleme is near the Nastapoka Island chain on the eastern shore of Hudson Bay in Canada. These islands form a beautifully circular arc with its centre of curvature out in the bay itself. If they are segments of an ancient rim, the crater must have been 275 miles in diameter, larger than all but a very few craters on the moon.