Beneath oceanic islands, the thickness (d) of a lens of fresh groundwater (density ρ‎_w) overlying sea water (density ρ‎_m) can be determined if the height above sea level (h) of the top layer of the lens is known and the conditions are static. The relationship is d = ah = ρ‎_w/(ρ‎_m – ρ‎_w) where a is typically about 38.