It is postulated that the collapse of the upper debris bed was the main cause of core failure and core material relocation to the lower vessel plenum during the Three Mile Island Unit 2 (TMI-2) accident. It is shown that this mechanism of core relocation can account for the time scale(s) and energy transfer rate inferred from plant instrumentation. Additional analysis suggests that the water in the lower half of the reactor vessel was subcooled at the onset of relocation. Moreover, subcooling serves to explain the final coolable configuration at the bottom of the TMI-2 vessel. An important aspect of the analysis of the relocation event is that significant quantities of corium melt were not mixed with water at any one time, indicating an even lower likelihood of steam-explosion-induced vessel and containment failure than heretofore believed.