At the end of the fifth and final match, Lee Sedol sat back quietly in his chair in a conference room at the Four Seasons Hotel in Seoul as the collected computer scientists celebrated around him. Lee, second only to fellow South Korean Lee Chang-Ho in international titles in the ancient Chinese board game of Go, put up a valiant fight against the machine, AlphaGo, created by Google's DeepMind division. AlphaGo had erred early on, but recovered to overpower the human and win the series four to one. Board games have been used since the early days of artificial intelligence research as ways to measure progress -- IBM's Deep Blue famously beat world chess champion Garry Kasparov in New York in 1997 -- and AlphaGo's victory marks another significant milestone in the advancement of the technology. Go presents a far greater challenge to AI than chess.