South Korea's Lee Sedol, the world's top Go player, waits after putting the first stone against Google's artificial intelligence program AlphaGo, as Google DeepMind's lead programmer Aja Huang, left, sits during the Google DeepMind Challenge Match in Seoul, South Korea, March 9, 2016. [Photo/Xinhua]

AlphaGo took the world by surprise as it defeated European Go champion Fan Hui in October last year. Some experts had predicted it would take decades of years for AI to defeat humans at the ancient Chinese board game.

Go, which originated in China more than 2,500 years ago, has been viewed as a grand challenge for AI due to its complexity and intuitive nature. It involves two players, who take turns putting markers on a grid-shaped board to gain more areas on it. One can occupy the markers of the opponent by surrounding the pieces of the other.