In Hollywood, they call it "within the frame."
They are showing us a partial image. The rest of the object extends beyond the frame of the photo. If they showed us the full sonar image of the object, it would reveal that it's just an ordinary boat, partially buried in the sediment at the bottom of the Baltic Sea.
But hey, it's the Baltic Sea.... a faraway and exotic place to most people, and as many comedy skits have demonstrated, anything is possible in such places.
_