"A proposal to split California into three states — Northern California, California, and Southern California — has qualified to appear on the ballot in November's general election." http://www.businessinsider.com/how-california-could-become-three-states-cal3-ballot-measure-2018-6 This is the first I've heard of this proposal.
It's the first step on a path to do so. CO had something similar not long ago. If you are wondering if this could actually happen, almost certainly not imo.