volcanic activity at yellowstone, one of the most geologically unstable places on earth. magna flows as close as five miles under the earth. powering geysers, steam vents and even earthquakes. recently yellowstone experienced a series of quakes over a short period. that is known as a swarm. >> yellowstone normally has lots of swarms. that's part of being in a volcanic system. what was interesting about the swarm that happened between december 2008 and january 2009 is that it was one with of the larger ones we've seen. >> reporter: so large, in fact, that a flurry of online discussions began, asking the question, is this swarm a sign of the next big volcanic eruption? scientists believe there have been at least three tremendous volcanic eruptions in the last 2.1 million years, the most recent 640,000 years ago. it it was massive.