The 1815 eruption of Tambora is one of the largest explosive volcanic events of the past 10 000 yr. By a conservative estimate, 175 km3 of nepheline-normative trachyandesitic pyroclastic material (equivalent to about 50 km3 of dense rock) was erupted in 24 h. Major activity began with a brief plinian phase followed by pyroclastic flows; much of the ignimbrite that was produced shows evidence of deposition by turbulent flows. A 6×7-km caldera, about 1 km deep, formed at the end of the pyroclastic flow phase; caldera volume closely matches that of magma ejected. Plinian and co-ignimbrite ash fall >1 cm thick covered >500 000 km2 of the Java Sea and surrounding islands, accounting for more than two-thirds of the magma volume erupted. This distal tephra is known only from 1815 reports, indicating that some great eruptions, especially those from island volcanoes, may be virtually undetectable in the geologic record without supporting information, such as deep-sea core data. Land-based volume estimates may be insufficient to characterize eruption magnitude in many cases.