Before the initial eruption 22,000 years ago there was a ‘wide and shallow basin nearly the same size of present Aira caldera [which] occupied the northern end of the Bay with an east-west ride' (Aramaki, S. 1984). The basin is separated from the rest of the bay by a ridge which is 300-500m above sea level. The topography encompasses the outline of an older caldera hence creating the assumption that there was a pyroclastic flow that predated the formation of present day Aira caldera.