Although Pinatubo’s eruption in 1991 caused nearly 900 deaths, many more lives would have been taken had it not been for the scientists who predicted the event.

The science of predictions involves noting changes in a volcano’s shape, tracking seismic activity, and monitoring emissions of water vapor and other gases. Increasingly high-technology tools have been used in recent years. A process known as seismic tomography uses x-ray technology to help locate magma miles below the Earth’s surface.

But as sophisticated as these techniques have become, predictions are still imprecise, and local residents are reluctant to evacuate unless they think danger is imminent. Sometimes that awareness arrives too late.

Volcanologists come from several scientific disciplines. Geophysicists and geochemists study the deepest subterranean causes of volcanoes. Geologists delve into their prehistoric records. Meteorologists observe the effects of dusts and gases as they burst into the atmosphere and sometimes travel around the globe.

Members of this international fraternity of scientists call themselves “living geologists.” Three of them, young American geologist Harry Glicken, and veterans Maurice and Katia Krafft of France, perished in the spring of 1991 while filming an eruption near the town of Shimabara, Japan. A heat cloud from an explosion of lava near the top of the volcano enveloped the trio. They died instantly, along with a number of Japanese journalists, cab drivers and farmers—in all, 43 more victims of the ancient furnace that smolders below.