THOSE living in the Philippines in June 1991 will never forget the eruption of Mount Pinatubo. It has become one of the events that mark the passage of time, like a death in the family or a war. Filipino history can be divided by it, the way Americans divide time by September 11, 2001, the way the world talks about before Covid and since Covid. The second largest eruption of the twentieth century, Pinatubo spit a cloud of ash 22 miles high into the atmosphere. For comparison, passenger jets fly at about six or seven miles. From the Philippines the ash cloud moved westward across the South China Sea and eventually drifted around the world several times. The ash settled several feet thick around the island of Luzon, which was a problem itself, but was made worse because, as luck would have it, a typhoon hit the island at the same time. Heavy rains soaked the ash and made a thick, heavy paste that collapsed buildings under its weight. In fact, more people were killed by collapsing buildings than directly by the volcano. Then came the mudslides, called lahars, which wiped out entire villages. In some areas the rain-soaked ash and lava flows settled into low areas 600 feet thick, countless acres of farmland forever buried under a new layer. Most airlines diverted their planes around the affected area, but with ash spread so widely it was almost impossible to avoid entirely. Jets flying near the cloud sucked ash into their engines, resulting in over $100 million in damage to aircraft. As we would expect, the impact on the environment and the global climate was enormous. Global temperatures dropped an average of one degree Fahrenheit and did not return to normal for years. Sulphur dioxide released in the eruption mixed with water and oxygen to make sulfuric acid, which rapidly deteriorated the ozone. The hole in the ozone layer above Antarctica reached its largest size following the eruption. Mount Pinatubo proved the value of early warning measures for volcanic activity. Thanks to small investments in equipment, scientists knew well in advance that conditions on the mountain were changing. Authorities were notified, evacuations were ordered, and tens of thousands of lives were saved. Even though hundreds died and hundreds of millions of dollars of damage resulted, it could have been much worse. In that sense, some solace can be taken from the Pinatubo disaster. Lessons were learned that have helped in developing even better early-warning measures for future events. Of course, that is small consolation for those who lost loved ones in the eruption. Dr. BC Cook taught history for thirty years and is a director and Pacific historian at Sealark Exploration (sealarkexploration.org). He currently lives in Hawaii.