April 18, 2013: New research out of the University of Utah shows the magma chamber under Yellowstone national park is bigger than previously thought. What's more, scientists believe the molten rock in this chamber is rising at an unprecedented rate displacing the surface crust. Knowing the size of the chamber and the rate at which magma rising can help gauge the size of a future eruption.
Past Yellowstone eruptions have buried much of the Midwest in tens of feet of ash and blotted out the sun for weeks. If Yellowstone were to have an eruption similar to its most recent blast, it could literally be the end of the civilization as we know it.
Read Unleashing Colter's Hell for my take on what might happen if this volcano were to erupt, and who would want that to happen.
Here is a great info graphic on the geology of the Yellowstone caldera.
Source: OurAmazingPlanet.com: Infographic: The Geology of Yellowstone