The term 'unfathomable deeps' refers to the depths of the ocean, which are so deep that they cannot be measured or understood. The term is often used to describe the unknown and mysterious nature of the ocean. The crossword solution for this term is 'abysms, ' which is a synonym for deep abysses or chasms. Abysms are often used to refer to the deepest parts of the ocean, which are still largely unexplored and mysterious.