During volcanic eruptions, molten magma and volcanic gases rise from an underground magma chamber, through a conduit, until they reach the crater's vent, from where the gases escape into the atmosphere and the magma is erupted as lava. A volcanic crater can be of large dimensions, and sometimes of great depth. During certain types of explosive eruptions, a volcano's magma chamber may empty enough for an area above it to subside, forming a type of larger depression known as a caldera. A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow volcanic crater lake which may also be called a maar. Explosion crater An explosion crater is produced by an explosion near or below the surface of the ground. A crater is formed by an explosive event through the displacement and ejection of material from the ground. It is typically bowl-shaped. High-pressure gas and shock waves cause three processes responsible for the creation of the crater, these being plastic deformation of the ground, projection of material (ejecta) from the ground by the explosion, and spallation of the ground surface. Two processes partially fill the crater back in, the immediate fall-back of ejecta, and later erosion and landslides of the crater lip and wall. The relative importance of the five processes varies, depending on the height above or depth below the ground surface at which the explosion occurs and on the composition of the ground. Differences in these characteristics will yield craters of different shapes, sizes, and other characteristics. Pit crater A pit crater (also called a subsidence crater or collapse crater) is a depression formed by a sinking or collapse of the surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. Pit craters are found on Mercury, Venus, Earth, Mars, and the Moon. Pit craters are often found in a series of aligned or offset chains and in these cases, the features is called a pit crater chain. Pit crater chains are distinguished from catenae or crater chains by their origin. When adjoining walls between pits in a pit crater chain collapse, they become troughs. In these cases, the craters may merge into a linear alignment and are commonly found along extensional