I’m almost certain, there are no jellyfishes big as this one, in the picture below. Never the less, I can’t seem to figure out how this illusion took place. It would be easy if the scuba diver was behind, and the jellyfish was just infront of the camera, then “relative sizes illusion” would be possible. However, it seems that the diver is infront of the jellyfish (or is he?). Can someone help me solve this?