Maar is a specific type of volcanic crater. They are created by powerful volcanic explosions which almost instantly create a circular hole which is later filled with groundwater. Groundwater is a very important component in this process as it is the reaction between rising hot magma and groundwater that gives so much explosive power to this process. Such a volcanic eruption is known as phreatomagmatic eruption. Created hole fills with water and becomes a crater lake.
Viti maar in northwestern Iceland is a superb location for hiking and photographing. This maar is geologically very young, dating from 1724.