- Link to Wikimapia: http://wikimapia.org/#y=50175057&x=6844783&z=15&l=0&m=h&v=2]] A maar is a broad, low relief crater that is caused by a phreatic eruption or explosion caused by groundwater contact with hot lava or magma. The maar typically fills with water to form a relatively shallow crater lake. The name comes from a local German dialect of Daun, which is in turn derived from Latin mare (sea). Maars are shallow, flat-floored craters that scientists interpret have formed above diatremes as a result of a violent expansion of magmatic gas or steam; deep erosion of a maar presumably would expose a diatreme. Maars range in size from 60 meters (200ft) to 2000m (6,500ft) across and from 10 meters (30ft) to 200 meters (650ft) deep, and most are commonly filled with water to form natural lakes. Most maars have low rims composed of a mixture of loose fragments of volcanic rocks and rocks torn from the walls of the diatreme. , in the US Stat