"It’s a sort of will-o-the-wisp thing," Glenn Seaborg once said of the discovery of plutonium. "We saw it and then it disappeared. Then we saw it again and then it disappeared and then finally we saw it and we could confirm it." That confirmation happened in Room 307 Gilman Hall, on February 23, 1941. It was a stormy night, Seaborg recalled. The 5 microgram sample of "element 94" had been bombarded in Ernest O. Lawrence’s 60-inch cyclotron.