President Harry S. Truman knew that one bomb would not be enough to force Japan to surrender, so he ordered two. What many don’t know is that there was a third bomb in reserve, just in case.

This third bomb had not been assembled yet, but its plutonium core—the heart of the bomb—was ready, and kept at the Los Alamos National Laboratory. When it became clear a third bomb would not be necessary, nuclear scientists at Los Alamos were delirious with excitement. Here there was in their hands, the rarest of the rare material—a 6.2 kg core of pure plutonium. They probed and prod the shinny metallic sphere and subjected it to countless experiments, until two sloppy scientists nearly blew up the laboratory and ruined it for every one. Both of them were dead within days, and the core acquired the nickname “demon core”.

A replica of the demon core in a mock setup used by one of the physicists during a fatal experiment.