During the first 9 months of 1982, more than 17,000 earthquakes have occurred within a very confined shallow crustal region of north central Arkansas. This is the largest earthquake swarm ever recorded in the central or eastern United States. Lifelong residents recall no previous activity, nor are there any instrumental records of earlier shocks. Although located in an area of considerable geological complexity, no prominent geological structure or geophysical anomalies can be associated with the swarm area. Why, then, is this earthquake swarm occurring, and why is it occurring in this particular location? If these questions ultimately can be answered, the results will have significant implications for attempts to quantify seismic hazards and identify potential seismic risk zones in the central and eastern U.S.