Diego Garcia is the above-water rim of a coral atoll composed of Holocene coral rubble and sand to the depth of about , overlaying Pleistocene limestone deposited at the then-sea level on top of a seamount rising about  from the floor of the Indian Ocean. The Holocene sediments are porous and completely saturated with sea water. Any rain falling on the above-water rim quickly percolates through the surface sand and encounters the salt water underneath. Diego Garcia is of sufficient width to minimise tidal fluctuations in the aquifer, and the rainfall (in excess of 102.5 inches/260 cm per year on average) is sufficient in amount and periodicity for the fresh water to form a series of convex, freshwater, Ghyben-Herzberg lenses floating on the heavier salt water in the saturated sediments.