Scientists have been debating for decades whether Mars once held a vast ocean covering a large part of its northern face. To prove the idea, theyve been looking for a bathtub ring - a distinct, level shoreline that shows where water once stood. But, despite years of looking, theyve only been able to find a very distorted potential shoreline whose height deviates by several kilometers - not exactly great evidence of a stable water level. But, according to a new paper in Nature from Abdallah Zaki and Michael Lamb of CalTech, what scientists should have been looking for wasnt a bathtub ring, but a continental shelf.