There is no standing fresh water on Caroline Island, but Nake Islet and South Islet have underground freshwater aquifers and freshwater lenses. The underground water supply on Nake attracted Polynesian settlers, but the first European explorers to reach the atoll searched in vain for drinking water; wells were built to tap drinking water for later temporary settlements. Little is known of the island's existing freshwater lenses. Soils are of poor quality, dominated by coral gravel and sand, with organic content present only within stable, forested island centers. Guano deposits make island soil, where it does exist, nitrogen-rich.