http://www.livescience.com/28821-yellowstone-supervolcano-bigger-plume.html

Yellowstone's Volcano Bigger Than Thought

"We are getting a much better understanding of the volcanic system of Yellowstone," said Jamie Farrell, a seismology graduate student at the University of Utah. "The magma reservoir is at least 50 percent larger than previously imaged."

Rest at link. They used something called 'geoelectric' imaging.

Brian

• Yellowstone's Volcano Bigger Than Thought