Mapanuepe lake is located at the confluence of Marella and Mapanuepe Rivers as the two rivers merge to become the Santo Tomas River. The subsequent rains following the 1991 eruption of Mount Pinatubo produced lahar that dumped volcanic debris on the Marella River, one of the major drainages of the mountain, aggrading the river that eventually dammed the Mapanuepe River. The rising waters submerged the Mapanuepe Valley including the barangays of Aglao (lower), Buhawen and Pili of San Marcelino, Zambales. During the development of the lake, breaching and reforming of the debris dam occurred following each lahar episode. At its maximum extent, the lake grew to an area of and had a stored water volume of 75 x 106 cubic meters () before reaching its current state. The only structure that remains is a church tower with a huge metal cross right in the middle of the lake. Prior to the 1991 eruption, studies of geologic formations and sediments of Mapanuepe Valley showed that the area was the site of a similar lahar-formed lake from old eruptions.