This element is one of many first made at UC Berkeley in the 1950s. Unlike most manufactured elements, though, this one is actually useful. This is how it starts up nuclear reactions.
Researchers first made californium in 1950. They didn't make much. The first cube of the element had sides 27 nanometres long. A human hair is 80,000 nanometres thick. The method used to make californium involved bombarding curium-242 with alpha particles (helium-4 nuclei) using the cyclotron. While that was good enough to prove the element could exist, it wasn't good enough to make a useful sample of it.
Usefulness came later, when bombarding plutonium-239 with single neutrons produced microgram-sized samples. More important than the increase in size was the change in isotopes. Early experiments produced californium-245, which has a seven minute half life. Californium-252, produced during the later experiments, has a half life of around two and a half years. During much of that time, a microgram-sized sample is giving off 170,000,000 neutrons per minute.
This makes it a very useful substance. Nuclear reactors work by getting neutrons shot into them, which causes some of the atoms to split, giving off more neutrons and perpetuating the reaction. The famous inanimate carbon rod can be used to soak up those extra neutrons, slowing the reaction down. When the reaction gets too slow, though, you need a neutron source to start it up. This is where californium-252 comes in. It's a tiny, effective neutron emitter that can jump-start a nuclear reactor.