Plastic pollution has become a worldwide problem. Monitoring is key in understanding how to prevent and clean the pollution. The focus has been on the most visible plastics, floating on the water surface or deposited on riverbanks. A large part of the problem may however reside below the water surface. Underwater measuring techniques are labour intensive, expensive and invasive. In our project RUMBA, we aim to reveal the hidden plastics using a new sound-based detection method. Improved understanding of the underwater plastic transport will help improve current mitigation strategies.