Knut the polar bear, a very popular resident of the Berlin Zoo, has died suddenly at the age of four (which is very young for a polar bear — in captivity, they can live into their 30s). Knut was famous for being hand-raised by zoo staff after his mother rejected him at birth. The cause of death was unclear; Knut reportedly went into the water in his compound, “had a short spasm and died.” Gruesomely, between 600 and 700 visitors were at his compound at the time and witnessed his death. Berlin — and the world! — mourns.