Associated Press - December 29, 2008 5:43 PM ET
CHEYENNE, Wyo. (AP) - Scientists are closely monitoring more than 250 small earthquakes that have occurred in Yellowstone National Park since Friday.
Swarms of small earthquakes happen frequently in Yellowstone. But Robert Smith, a professor of geophysics at the University of Utah, says it's very unusual to have so many over several days.
The largest tremor was Saturday and measured magnitude 3.8.
Smith says it's hard to say what might be causing the tremors but notes that Yellowstone is very geologically active. An active volcano there last erupted 70,000 years ago.