On this day in 1991 at Clark Air Base in the Philippines, 14,500 personnel are evacuated in anticipation of the eruption of Mount Pinatubo. Over the next several days, the eruptions killed hundreds of people and sent tons of ash and sulfur dioxide into the atmosphere.

In the early 1990s, about 30,000 people lived very close to Mount Pinatubo, which is 60 miles north of the capital city of Manila. The volcanic mud that makes up the lower slopes provided good soil for agriculture. The two major United States military facilities in the Philippines, Clark Air Base and Subic Bay Naval Station, are only 15 and 25 miles away, respectively, from the volcano.

Pinatubo is a stratocone, a volcanic mountain that forms a dome made of lava. (Mt. Fuji in Japan is another example of this type of mountain.) It was 5,725 feet high at the time of the 1991 eruptions. The first indications of activity on the volcano were noticed on April 2, and in mid-May, scientists discovered that sulfur dioxide levels were 10 times above normal at Pinatubo, a sign of an impending eruption. Although the timing of the eruption could not be predicted, the readings raised alarms and emergency preparations began.

Evacuation of the surrounding area began on June 7. On the morning of June 12, the first major explosion occurred, blasting ash 62,000 feet in the air and destroying part of the mountain’s dome. The eruption continued on and off for the next day and then, on the afternoon of June 14, another big blast spread gas and ash miles away. The final eruption took place the following morning, spewing 20 million tons of sulfur dioxide into the air. Approximately 350 people were killed by the toxic emissions. The early warning and preparations saved thousands of lives.

The mountain lost nearly 1,000 feet in the eruptions—it now stands at just 4,800 feet high.