The Victoria Island structure is a buried, bowl-shaped geological feature in the Sacramento–San Joaquin River Delta west of Stockton, California, regarded as a probable impact crater. About 5.5 kilometers, or 3 miles, across, it lies deep within shale sediments that once formed a sea bed, with its top roughly 1,490 to 1,600 meters below sea level, leaving no visible surface expression. Discovered during oil exploration and publicly reported in March 2007, it is thought to have formed between 37 and 49 million years ago. Because its exact surface position is not more precisely published than Victoria Island, it is known chiefly through subsurface geological data.