One of the many pleasant moments in the Djiboutian trip was for me to try out a lens with a focal length of 8 mm under water. In photographic simplicity, this lens is called the "circular". This is a kind of "fish eye", which has a viewing angle of 180 degrees or more, but does not fill the whole frame and gives the image in the form of a correct circle.
My friend, when he saw the first pictures taken by me underwater with the help of this lens, said that in his childhood, when he dreamed of swimming on a submarine, that's how he imagined underwater landscapes through its round portholes.