 Complete the following phrase:  Another empirical method to study the likelihood of certain natural risks is to investigate the geological record. For example, a comet or asteroid impact event sufficient in scale to cause an impact winter that would cause human extinction before the year 2100 has been estimated at one-in-a-million. Moreover, large supervolcano eruptions may cause a volcanic winter that could endanger the survival of humanity. The geological record suggests that supervolcanic eruptions are estimated to occur on average about every 50,000 years, though most such eruptions would not reach the scale required to cause human extinction. Famously, the supervolcano 