At 1,745 meters above sea level, Pinatubo had little topographic prominence. Dense forests obscured its slopes, and the surrounding peaks of the Cabusilan sub-range hid it from most vantage points. It sits about 87 kilometers northwest of Manila, on the tripoint where Zambales, Tarlac, and Pampanga provinces meet. Clark Air Base, maintained by the United States, lay just 14 kilometers east of the summit. The U.S. Naval Base at Subic Bay was 37 kilometers to the south. Some six million people lived within range of its effects. The volcano belongs to a chain formed by the Eurasian Plate sliding under the Philippine Mobile Belt along the Manila Trench, the same subduction process that has built volcanoes along the western edge of Luzon for millions of years. Ancestral Pinatubo began erupting roughly 1.1 million years ago, and its most explosive ancient eruption was estimated to be five times larger than the 1991 event.

Warning signs began in March 1991, when magma rising from more than 32 kilometers below triggered thousands of small earthquakes and steam explosions that blasted three new craters on the north flank. Sulfur dioxide poured from the vents. Scientists from the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey issued warnings that led to the evacuation of tens of thousands of people, a decision that saved countless lives. On June 12, a small blast at 3:41 a.m. marked the start of the violent phase. Massive eruption columns reached heights of 19 kilometers, then 24 kilometers, generating pyroclastic surges that raced four kilometers down river valleys. Friction in the ash column produced volcanic lightning. The climactic eruption on June 15 coincided with Typhoon Yunya making landfall, mixing ash with rain in a lethal slurry. The day of darkness stretched 36 hours and was later called Black Saturday. The eruption left a caldera where the summit had been and launched pyroclastic flows, falls, and lahars that buried towns and altered river systems for years.

The eruption's consequences reached far beyond Luzon. Pinatubo ejected more particulate matter into the stratosphere than any eruption since Krakatoa in 1883. Aerosols formed a global layer of sulfuric acid haze that circled the planet. Global temperatures dropped by about 0.5 degrees Celsius between 1991 and 1993, and ozone depletion temporarily increased. Clark Air Base, one of the largest American military installations in Asia, was buried under ash and abandoned. Subic Bay Naval Base was also severely affected. The destruction accelerated the closure of both bases, ending nearly a century of American military presence in the Philippines. Rivers choked with volcanic debris spawned lahars during every monsoon season, threatening communities for years after the eruption itself.

The Aeta people call the mountain the abode of Apo Namalyari, the Lord of Happenings, a deity revered by the Sambal, Aeta, and Kapampangan peoples of the Zambales range. An ancient legend tells of Bacobaco, a spirit of the sea who could transform into a giant turtle that breathed fire. When chased by spirit hunters, Bacobaco fled to the mountain and tore open its summit, showering the land with rock, mud, dust, and fire for three days while howling so loudly the earth shook. The word pinatubo itself, in Sambal and Tagalog, may mean either a fertile place where one can make crops grow or simply made to grow, possibly reflecting folk memory of an earlier eruption around 1500 AD. After 1991, the eruption displaced thousands of Aeta families. Beginning in 2009, the Philippine government granted Certificates of Ancestral Domain Title covering more than 23,000 hectares on both sides of the volcano. The Aeta reclaimed their lutan tua, their ancestral land, including the summit and the lake that now fills it.

The caldera and its lake have become a popular destination for hikers and adventurers, with the preferred route running through Barangay Santa Juliana in Capas, Tarlac. The volcano remains active and monitored by PHIVOLCS; seismic activity increased in early 2021, and a weak phreatic explosion occurred that November. In 2025, Aeta communities protested their exclusion from tourism revenues on their own ancestral land, calling attention to a tension that the eruption set in motion decades ago. The mountain that the world forgot, then feared, then marveled at, continues to reshape the lives and landscape around it. Its caldera inspired the design of the New Clark City Athletics Stadium in Capas, and Philippine president Ramon Magsaysay, a Zambales native, once named his presidential C-47 aircraft Mt. Pinatubo. That plane crashed into Mount Manunggal in Cebu in 1957, killing the president and 24 others, a grim coincidence that the mountain's name seems to attract drama across the decades.