On the other hand, meteorite impact craters are not formed by earth-processes at all. As we have seen, they result when large bodies of matter from the regions of space _outside_ the earth chance to strike the surface of our planet at high speed. The study of meteorite craters is therefore a special field. It is also one of quite recent development; not until 1905 was the first meteorite crater recognized as such. The first thing to be said on this subject is, of course, that not all holes in the ground, however large and impressive, were necessarily formed by the impact of meteorites. Features that resemble meteorite craters may result from certain ordinary earth-processes. For example, the rock layers underlying a particular area may be dissolved away by waters circulating beneath the surface of the ground. The overlying crust will eventually collapse into the empty space, and what geologists call a “sink hole” or a “sink” is formed. Many such sinks surround the genuine meteorite crater near Odessa, Texas, and at times have been mistaken for the real thing. Since there is some possibility of confusion about whether or not a hole in the ground is a meteorite crater, it is comforting to know that scientists have come up with a handy set of rules for reaching a decision on this point. These rules can be stated in the form of several questions that crater-investigators should ask themselves: Have you found meteorites in or near the crater-like feature? In its vicinity, have you found pieces of country rock that show the effects of high temperature and pressure (melting or crushing)? Did people actually see a meteorite come to earth at the point where the crater is located and where, to their certain knowledge, no crater existed before? If the answer to all—or even one—of these questions is yes, then it is quite likely that the crater-like feature is actually a meteorite crater. Naturally, if the answer to the _first_ question is yes, the matter is practically settled in favor of the meteoritic origin of the feature. If the impact has taken place in horizontally bedded rock strata—that is, in flat beds of rock lying one on top of another like the layers in a stack of griddle cakes—a meteorite crater will have a characteristic _rim_ of upturned or even overturned rock layers. (None of the ordinary sink holes near the Odessa crater show such rims.) In addition, pieces of rock shattered and thrown out by the impact will be found in all directions around the crater. The amount and size of this fragmented material will decrease with distance outward from the crater.