The "Toba eruption" (the "Toba event") occurred at what is now Lake Toba about 75,000±900 years ago. It was the last in a series of at least four caldera-forming eruptions at this location, with earlier calderas having formed around 788,000±2,200 years ago. This last eruption had an estimated VEI=8, making it the largest-known explosive volcanic eruption within the last 25 million years. Bill Rose and Craig Chesner of Michigan Technological University have estimated that the total amount of material released in the eruption was about —about of ignimbrite that flowed over the ground, and approximately that fell as ash mostly to the west. However, based on the new method (crystal concentration and exponential), Toba possibly erupted of ignimbrite and co-ignimbrite. The pyroclastic flows of the eruption destroyed an area of least , with ash deposits as thick as by the main vent. The eruption was large enough to have deposited an ash layer approximately thick over all of South Asia; at one site in central India, the Toba ash layer today is up to thick and parts of Malaysia were covered with of ash fall. In addition it has been variously calculated that of sulfurous acid or of sulfur dioxide were ejected into the atmosphere by the event. The subsequent collapse formed a caldera that filled with water, creating Lake Toba. The island in the center of the lake is formed by a resurgent dome. The exact year of the eruption is unknown, but the pattern of ash deposits suggests that it occurred during the northern summer because only the summer monsoon could have deposited Toba ashfall in the South China Sea. The eruption lasted perhaps two weeks, and the ensuing volcanic winter resulted in a decrease in average global temperatures by for several years. Ice cores from Greenland record a pulse of starkly reduced levels of organic carbon sequestration. Very few plants or animals in southeast Asia would have survived, and it is possible that the eruption caused a planet-wide die-off. However, the global cooling has been discussed by Rampino and Self. Their conclusion is that the cooling had already started before Toba's eruption. This conclusion was supported by Lane and Zielinski who studied the lake-core from Africa and GISP2. They concluded that there was no volcanic winter after Toba eruption and that high H2SO4 deposits do not cause long-term effects. Evidence from studies of mitochondrial DNA suggests that humans may have passed through a genetic bottleneck around this time that reduced genetic diversity below what would be expected given the age of the species.