Concurrently with the installation of a detailed seismic network around Mount Pinatubo in April-May 1991, Philippine and USGS scientists undertook a rapid geologic reconnaissance of the volcano to determine the style and extent of Pinatubo's past eruptions. This reconnaissance showed that in the past few thousand years, the volcano's activity has been dominated by large explosive eruptions, each of which generated voluminous pyroclastic flows. In addition, lahar deposits along river valleys extending far beyond the foot of the volcano indicated that erosion of pyroclastic-flow deposits had generated lahars and floods that redistributed the pyroclastic debris over the lowlands surrounding Pinatubo. Their geologic mapping led to the creation of a volcano-hazard zonation map which proved to be remarkably accurate, helping to save thousands of lives during the cataclysmic eruption on June 15, 1991.