A world champion has retired because he has decided humans cannot beat computers at his game. The grand champion is Lee Se-Dol from South Korea. He was world champion at the ancient, strategy board game Go. Some people compare Go to chess. Mr Lee is the only person in the world to ever beat Google's AlphaGo computer algorithm. This is a special A.I. computer program created by Google to play Go. Mr Lee, an 18-time world champion, told reporters earlier this week that: "Even if I become the number one, there is an entity that cannot be defeated. With the debut of A.I. in Go games, I've realized that I'm not at the top even if I become the number one."

The game of Go originated in China around 3,000 years ago. Today, it is played mostly in China, Japan and South Korea. It is believed to be the oldest board game in the world. In 2016, the International Go Federation said it had 75 member nations, and that over 46 million people worldwide knew how to play Go. Mr Lee started playing at the age of five. He turned professional seven years later, when he was 12. He played five games against Google's AlphaGo and lost four of them. He said his one victory was because of a "bug" in the program. The bug did not know what to do when Mr Lee made a "tricky" move. Today's Go programs are much more powerful than the one that beat Mr Lee.