Google’s AlphaGo, an artificial-intelligence program, beat South Korean grandmaster Lee Sedol at Chinese board game Go on Tuesday, winning its fourth match in the five-game series. The program made history in 2015 after beating a human pro player, Fan Hui, in a 5-0 match. But 33-year-old Lee was believed to be a much more difficult and creative opponent. AlphaGo had claimed the series win with its third victory Saturday, which DeepMind founder Demis Hassabis called: “One of the most incredible games ever.” He added: “To come back from the initial big mistake against Lee Sedol was mind-blowing!” Lee did win the fourth match against AlphaGo, despite the fact that he was bested in the final game. Go is much more difficult than chess, making it ideal for AI researchers—IBM’s AI, Deep Blue, famously beat former world chess champion Garry Kasparov in 1997 by brute-force calculations. In Go, there are simply too many moves for a machine to win that way.