A computer program winning a Go tournament.

In chess, human grandmasters routinely beat the best computers, but changing that was simply a matter of faster processors and larger memory, problems solvable by the application of sufficient quantities of money. In principle the game was already solved, and within a few decades, would be solved in practice as well.

Go was considered a much harder problem. Programs of similar complexity to a decent chess program couldn’t even look at a finished game between go pros and reliably say who won, let alone get there itself. Well, guess what?