I remember reading a few years back a news article where scientists picked up on sonar a massive moving object in the pacific ocean they believe it was the size of four football fields in diameter and moving at 100knots or more deep under the ocean. They never could figure out what it was just that whatever it was it was big and moving fast, sending in a sub to investigate was out of the question cause of the speed and depth it was at. Then after about an hr of tracking it, it disappeared. A mothership

Or maybe a large school of fish high on some toxic seaweed speeding through the depths, or possibly a sea monster

. Who knows!