Only six players have ever recorded 59s in Tour events (counting the 2 on the Nationwide, and Annika.). Last Wednesday, Mickelson became number 6. Amazing.
I’ve heard it said that a 54 (18 under par) would be the perfect score. I think we’ll see one of those in the not too distant future. It’ll be on an older course that will be overwhelmed by one of the longer hitters, who will be hitting driver, wedge on all of the par 4s.