Some risks are due to phenomena that have occurred in earth 's past and left a geological record . Together with contemporary observations , it is possible to make informed estimates of the likelihood such events will occur in the future . For example , an extinction-level comet or asteroid impact event before the year 2100 has been estimated at one-in-a-million . Supervolcanoes are another example . There are several known supervolcanos , including Mt. Toba , which some say almost wiped out humanity at the time of its last eruption . The geologic record suggests this particular supervolcano re-erupts about every 50,000 years
some say
Toba catastrophe theory
Global catastrophic risk