You start over. That is why this is a masterpiece. He had to find a slab of marble that big without flaws. Internal flaws could not be detected at that time, so it was a huge gamble. His genius comes from his ability to "see" the finished product in his mind and also possess the skills to remove the marble so that only his vision remained. He did all this with a hammer and chisel. What you don't see in most photos is the [amazing detail that he achieved in his works.](https://www.boredpanda.com/michelangelo-david-close-up-photos/?utm_source=duckduckgo&utm_medium=referral&utm_campaign=organic)