Along California's coastline, creatures smaller than a fingernail have been quietly performing geological work that rivals the state's rivers. Sand hoppers — amphipod crustaceans long dismissed as coastal footnotes — move tons of sediment annually, and new research suggests they may displace more sand than any other animal on Earth. Their discovery as major architects of beach formation invites a humbling question that echoes far beyond the shoreline: how much of the natural world has been shaping us while we weren't paying attention?