A chunk that I always pass through near my base in my single player world now is a giant hole in the ground. At first I thought it was a visual error, but I can break and place blocks inside it. A torch I previously had there also got replaced but on the hole's floor. Fish have also spawned within the water at the floor. This leads me to believe it regenerated. The only thing I did was save the world yesterday, and reloaded it today. There are similar problems in the forums to this but for much older versions of MC.