When people pump too much water from coastal aquifers, saltwater can rush in. Thick with salt and minerals, seawater is denser than freshwater; once in an aquifer, there is no known way of flushing it out. Coastal aquifers are imperiled from Maine to Florida; on the Arabian coast; in the suburbs of Jakarta (metropolitan population, more than 10 million); throughout the Mediterranean; and in a host of other places.