When a volcano erupts, stuff comes out of the ground. Most of the time, it isn't much stuff (and by "stuff", we're talking lava, ash, volcanic gases in varying proportions). But sometimes it's a lot.

Add Yahoo as a preferred source to see more of our stories on Google. The Yellowstone Caldera, spanning Wyoming, Idaho, and Montana, is among the most seismically active volcanic regions on Earth. A ...

The Yellowstone Caldera, spanning Wyoming, Idaho, and Montana, is among the most seismically active volcanic regions on Earth. A caldera forms when a volcano erupts, emptying the underlying magma ...