The nature of the submarine deposits produced by the 1883 Krakatau eruption in Indonesia has remained controversial for more than a century. Knowledge of their character, however, is critical to understanding the origin of the devastating tsunamis generated during the event, which claimed about 36,000 lives. Results of a 1990 marine geological investigation reported here demonstrate that the deposits are largely of pyroclastic flow origin. Pyroclastic flows are partially fluidized mixtures of particles and gases that travel up to 150 m/s and have internal temperatures as high as 600°C. They are denser than the atmosphere but are likely to be comparable to seawater in density. What happens when such a flow encounters the sea along the coast of an active volcano has been the subject of much speculation [Cas and Wright, 1987] but little research [Sparks et al., 1980a, 1980b]. One well-known effect is the generation of tsunamis [Kienle et al., 1987] and indeed, 20% of all volcanogenic tsunamis have been attributed to the entrance of pyroclastic flows into the sea [Latter, 1981]. The most devastating of these occurred during the 1883 eruption of Krakatau volcano.