The 1991 eruption of Mount Pinatubo, in the Philippines, was the second largest eruption of the 20th century, with a volcanic explosivity index rating of 6. That puts it somewhere in Krakatoa territory: bigger than Mount St. Helens' in 1980, but smaller than Tambora in 1815. (You know, for those of you who follow this volcano stuff.) Pinatubo finally exploded on June 15, tossing out some two and a half cubic miles of material, including rock, ash and toxic fumes. Some 800 people died as a result of the eruption, which left a mile-wide crater lake at the summit.