Crater Lakes/Caldera/Maars: MAIN CRATER LAKE - 1.9 km in diameter; blue-green in color, 4 m above sea level, deepest point: 76 m Taal Volcano Island has 47 craters and 4 maars TAAL CALDERA - 25 km across and formed between 140,000 to 5,380 BP TAAL LAKE - inside the caldera; 267 sq. km and 2 m above sea level A maar is a low-relief, broad volcanic crater formed by shallow explosive eruptions. The explosions are usually caused by the heating and boiling of groundwater when magma invades the groundwater table. Maars often fill with water to form a lake.