In a nuclear reactor, the energy stored in graphite as a result of the Wigner effect. When released spontaneously, as it is wont to do, it creates heat. Unwanted heat. Heat that can melt fuel, ignite uranium, and release fission products into the atmosphere. Or so predicted Eugene Wigner in 1949. Eight years later, at the Windscale Reactor No. 1, British nuclear scientists got to see firsthand this worst case scenario.