Ilopango caldera in central El Salvador erupted violently to produce widespread ashfalls and hot ash flows at least four times in the last 60,000 years, and most recently in AD 260. Ilopango is perhaps the most dangerous volcanic center in Central America because of its potential for violent explosive eruption and its proximity to San Salvador, the largest city in Central America. In AD 260, hot ash flows spread throughout the Salvadoran highlands and swept up to 40 km down steep valleys onto the Pacific coastal plain, areas then populated by early Mayan civilization and now densely populated. Ash fell across much of Central America, much of southern Mexico, and nearby parts of the Pacific Ocean and Gulf of Mexico. The eruption may well have been the greatest catastrophe ever to befall Meso-American civilization. The eruptives have total volume of 20 to 50 cubic km and encompass 6 stages: (1) initial fall and surges, (2) main Plinian fall, (3) energetic pyroclastic deposit, (4) phreatoplinian fall, (5) bedded phreatomagmatic fall, and (6) main ignimbrite. This study will characterize controls on eruption style and intensity and differentiate factors intrinsic and extrinsic to the magma. Study of the complex but distinctive stratigraphy of deposits will address the following important topics in volcanology: field calibration of numerical models for Plinian eruptions, transport and deposition mechanisms for pyroclastic flows and surges, and magma-water interactions in explosive eruptions through shallow water. The chief goal of the study is a better understanding of the behavior of explosive eruptions through shallow water and, specifically, that of Ilopango caldera during eruptive cycles. An important secondary goal is increasing awareness of volcanic processes and their affiliated hazards in El Salvador.