Maar is a crater produced by minor, short-lived natural explosion. Such explosion is mostly volcanic. Maars are circular pits, which are commonly occupied by water. Thus maars are generally lakes. The most notable area of maars is the Eifel region of Germany, west of the Rhine, where the maars are generally lakes about half to three-fourths of a kilometer across.

The maars are surrounded by a low ring of debris, which consists of the ‘exploded bedrock’, and a little volcanic matter. Cotton regards the maars of Eifel as the true maars because they are located in non-volcanic terrain and the explosions responsible for them exploded only a small quantity of debris. Occurring in non-volcanic terrains, most true maars are believed to be due to phreatic explosions. Among the volcanic gases steam is dominant.

Such steam may be originally contained in the magma. It may be derived from the water of lakes or ground water or marine water when magma may come in contact with such water. Explosion, which is due to the heating of such water and not due to the eruption of burning debris, is called phreatic. But it is not possible to ascertain how much of such steam is phreatic and how much rnagmatic. Such volcanic action, which may remain concealed within the crust without finding expression above the surface of the earth except as steam-propelled explosion, has been called ‘cryptovolcanic’. After the formation of the depression, there has been no activity in this feature, which has been termed as ‘abortive’ volcano.