Pluto

On the night of February 23, 1941, Glenn Seaborg, Edwin McMillan, Joseph Kennedy, and Arthur Wahl bombarded uranium-238 with deuterons in the 60-inch cyclotron at the University of California, Berkeley. They produced element 94. McMillan had already named element 93 neptunium, after Neptune — the planet beyond Uranus. Seaborg continued the planetary sequence and called element 94 plutonium, after Pluto. The planet had been discovered only eleven years earlier by Clyde Tombaugh.

Seaborg later admitted he had considered the name plutium, which would have been more consistent with other element names. But plutonium sounded better, and it gave the element the chemical symbol Pu — which Seaborg and his colleagues found amusing, as it sounded like a child's exclamation of disgust. The most destructive element ever created was given its symbol as a private joke by young physicists.

Plutonium became the core of the Manhattan Project. The Trinity test on July 16, 1945, was a plutonium device. Fat Man, the bomb dropped on Nagasaki on August 9, 1945, was a plutonium bomb. Unlike uranium, plutonium does not exist in nature in any meaningful quantity — it is entirely a human creation, synthesized in reactors. Humanity manufactured its own doom material and named it after the Roman god of the dead.

The naming turned out to be darker than anyone intended. Pluto was Hades, lord of the underworld, ruler of the dead. Plutonium's half-life is 24,100 years — any plutonium produced today will remain lethal until approximately the year 26,000. Seaborg chose the name for the sake of planetary sequence and a good joke. He accidentally created the most ominous name in the periodic table: the element of the dead, deadly for longer than recorded civilization has existed.