Actually, that's the shadow of the USS Arkansas.
Arkansas was the closest ship to the bomb other than the ship from which it was suspended. The underwater shock wave crushed the hull on the starboard side facing the balst, and rolled the battleship over onto its port side. It also ripped off the two starboard side propellers and their shafts, along with the rudder and part of the stern, shortening the hull by 8 meters. Contrary to popular belief, Arkansas was not lifted vertically by the blast., the dark area is caused by Arkansas’s hull interfering with the development of the spray column, creating a hole in the plume. Forensic examination of the wreck during multiple surveys since the test conclusively show that structural failure of hull plating along the starboard side allowed rapid flooding and capsized the ship.
