Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is 42 km (26 mi) west of the dormant and solitary Mount Arayat. Before 1991, the mountain has no recorded historical eruptions. It was heavily eroded, inconspicuous and obscured from view. It was covered with dense forest which supported a population of several thousand indigenous people, the Aeta, who had fled to the mountains from the lowlands during the Spanish conquest of the Philippines.

The volcano's Plinian / Ultra-Plinian eruption on June 15, 1991 produced the second largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in the Alaska Peninsula. The colossal 1991 eruption had a Volcanic Explosivity Index (VEI) of 6, and came some 450–500 years after the volcano's last known eruptive activity (estimated as VEI 5, the level of the 1980 eruption of Mount St. Helens), and some 1000 years after previous VEI 6 eruptive activity. A VEI of 6 corresponds to 10 to 100 cubic km of released material (Pinatubo released an estimated 6 to 16 cubic km of ash). Successful predictions of the onset of the climactic eruption led to the evacuation of tens of thousands of people from the surrounding areas, saving many lives, but surrounding areas were severely damaged by pyroclastic flows, ash deposits, and later by lahars caused by rainwater remobilizing earlier volcanic deposits; thousands of houses and other buildings were destroyed.