The people of Easter Island appeared to drink directly from the sea, European explorers reported in the late 1700s. And today, you can see animals—most famously horses—do the same thing. What's going on? While surrounded by a vast ocean, fresh water is a scarce commodity on Rapa Nui, as the island is known to its native inhabitants. No rivers or streams cross its surface, and it sports only three small crater lakes that can dry up during periodic droughts. Due to a quirk of geology, rainwater immediately sinks down through the porous bedrock, where it feeds an underground aquifer, explained...