Initial results of the first scientific expedition to the caldera are presented, noting that the caldera was formed during a major eruption 2.5 m. y. ago when 1000 cu km of ignimbrite were erupted. The prewelding thickness of ignimbrite within the resurgent part of the caldera is approximately 2 km. The ignimbrite is fine-grained and devoid of large pumice and lithic clasts. It is noted that an underlying plinian air fall pumice deposit is absent. The wide dispersal of the ignimbrite is seen as indicating an unusually violent eruption, perhaps initiated by subsidence of the caldera floor into the magma chamber.