Sand from different beaches has its own unique sound—and researchers have figured out how to listen in, The Economist reports. Sand, which is a valuable commodity, is often taken illegally for construction projects and artificial beaches. (India even has its own “sand mafia.”) To find out where a sample of sand comes from, scientists drop it in acid, which breaks down the sand’s carbonate chemicals (made from the shells of long-dead sea creatures) into carbon dioxide bubbles. Using sensitive listening devices, the researchers measure how the bubbles change the way sound travels through the mixture—yielding a unique frequency. When scientists listened to sand samples from several different beaches in the Netherlands, they found that each had a unique signature, they report in Applied Acoustics. Future research might address a more pressing philosophical question: If sand falls in acid with no one there to hear it, does it still make a sound?