A long-feared monster earthquake off California, Oregon and Washington could cause some coastal areas to sink by more than 6 feet, dramatically heightening the risk of flooding and radically reshaping the region with little to no warning. Those are the findings of a new study that examined the repercussions of a massive earthquake on the Cascadia subduction zone, which stretches from Northern California up to Canada’s Vancouver Island. The study, published Monday in the journal Proceedings of the National Academy of Sciences, concluded that in an earthquake scenario with the highest level of subsidence, or land sink, the area at risk of flooding would expand by 116 square miles, a swath that’s 2½ times the size of San Francisco.