Preeruption Mount Pinatubo, April 16, 1991. View from the northwest, up the Maraunot River valley. The river had become acidic and silty, owing to reactivation of the hydrothermal system and phreatic explosions of April 2, 1991 (the vents of which were just out of view at left edge of photograph). Steam was from 2-week-old fumaroles on the upper north slope of the volcano. The fumarole farthest to the right (behind a jagged ridge, right of the one visible on the valley floor) would later become the site of the preclimactic lava dome extrusion of June 7-12 (Hoblitt, Wolfe, and others, this volume). Mount Negron is behind and to the right of Pinatubo. (R.S. Punongbayan)