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A complex crater is an impact crater in which the rearrangement of material involves more than simply throwing away some material, i.e., the crater is more than just a ring-shaped mound. The common such feature is a high point in the center (central peak), so seen from above, it looks like a circle with a dot in the middle (a central-peak crater). Such craters are formed by larger impacts, generally being the larger craters. The minimum size of such craters depends on the gravity of the astronomical body, the more gravity, the smaller the minimum complex craters: Earth sees complex craters as small as 2 km diameter whereas on the Moon, the minimum is 20 km. The mechanism behind complex craters is still a topic of research. The term simple crater is sometimes used to indicate a crater is not complex.