Descending would have been helped by a negative Archimedes Force, i.e. being denser than water. Of course, this would have had the disadvantage of hampering coming up again. Young plesiosaurs show pachyostosis, an extreme density of the bone tissue, which might have increased relative weight. Adult individuals have more spongy bone. Gastroliths have been suggested as a method to increase weight or even as means to attain neutral buoyancy, swallowing or spitting them out again as needed. They might also have been used to increase stability.