In 2011, water pollution closed or drove visitors away from U.S. shores on more than 15,000 beach days across the country. In many places, the problem is getting worse. As coastal towns crowd with rooftops and parking lots, they produce more runoff from rain. The runoff picks up bacteria from animal waste and collects in pipes that then release the water into the ocean. Pretty gross. But some engineers have a simple and effective solution: Send runoff underneath the beach instead, where sand can filter the bacteria out.