When even more highly gas-charged magma reached Pinatubo’s surface on June 15, the volcano exploded in a cataclysmic eruption that ejected more than of material. The ash cloud from this climactic eruption rose into the atmosphere. At lower altitudes, the volcanic ash was blown in all directions by the intense cyclonic winds of a coincidentally occurring typhoon, and winds at higher altitudes blew the ash southwestward. A blanket of ash and larger pumice lapilli blanketed the countryside. Fine ash fell as far away as the Indian Ocean and satellites tracked the ash cloud several times around the globe. Huge pyroclastic flows roared down the flanks of Mount Pinatubo, filling once-deep valleys with fresh volcanic deposits as much as thick. The eruption removed so much magma and rock from below the volcano that the summit collapsed to form a wide caldera. Following the climactic eruption of June 15, 1991, activity at the volcano continued at a much lower level, with continuous ash eruptions lasting until August 1991 and episodic eruptions continuing for another month. Activity at the volcano remained low until July 1992 when a new lava dome started growing in the caldera. Volcanologists suspected that further violent eruptions could be possible, and some areas were evacuated. However, the eruption was only minor. The last eruption from Mount Pinatubo took place in 1993. The 1991 caldera afterwards filled with water from annual monsoon rains and a crater lake, Lake Pinatubo, was formed. In 1992, a growing lava dome formed an island, which was eventually submerged by the lake. Initially, the lake was hot and highly acidic, with a minimum pH of 2 and a temperature of about . Subsequent rainfall cooled and diluted the lake, lowering the temperature to and raising the pH to 5.5 by 2003. The lake deepened by about per month on average, eventually submerging the lava dome, until September 2001, when fears that the walls of the crater might be unstable prompted the Philippine government to order a controlled draining of the lake. An estimated 9,000 people were once again evacuated from surrounding areas in case a large flood was accidentally triggered. Workers cut a notch in the crater rim and drained about a quarter of the lake's volume. On July 10, 2002, the west wall of the crater collapsed, slowly releasing approximately of water and sediments into the Maraunot River in Botolan, Zambales. On July 26, 2011, a 5.9 magnitude earthquake struck close to Pinatubo; however, no major damages or casualties were reported.