The Yellowstone 'super volcano' deposited six foot of ash on Nebraska, 600,000 years ago. The 'super volcano' Toba blew 70,000 years ago and some estimate that as few as 5,000 humans worldwide survived. (this is one cause of the DNA 'bottleneck' we hear so much about). Also, Haiwaai(sp) has a large crack and a history of 'long run' land slides. So....