The Yellowstone caldera is no typical volcano. Its elongated form measures about 35 miles by 45 miles, considerably larger than most. Yellowstone Lake stands at the center of the caldera and shows evidence of volcanic activity that has formed some of its arms. Yellowstone contains one of the world’s largest geothermal systems.
The caldera has generated large amounts of ash over geologic history. One 12-million-year-old deposit of Yellowstone ash at Ashfall State Park in Nebraska entombed rhinoceros, horses, camels and birds that had gathered around a watering hole and today provide paleontologists with a deep view of ancient ecology.
For links to recent scientific reports about the caldera, see this page on Volcano World at Oregon State.