"It's sheer mathematics," Professor Murray Shanahan, an AI researcher at Imperial College London, told BuzzFeed News. "The number of possible board configurations in chess, of course, is huge. But with Go, it's enormously larger."

In chess, there are on average about 35 to 38 moves you can make at any point. That's called the "branching factor". In Go, the branching factor is about 250. In two moves, there would be 250 times 250 possible moves, or 62,500. Three moves would be 250 times 250 times 250, or 15,625,000. Games of Go often last for hundreds of moves.

It's sometimes said that in chess there are more possible games than there are atoms in the observable universe. In Go, by one estimate, there are something like a trillion trillion trillion trillion trillion trillion trillion more than that. To write the total number out, you'd need to put a 1 followed by 170 zeroes. That's why, nearly 20 years after computers became better at chess than humans, they've only just caught up at Go.