The Zwin is a former sound that is now silted up and lies between the resort of Knokke-Heist and the Dutch border. It once served the ports of Sluis, Damme and Bruges. Now a nature reserve (part of which is open to the public), it is home to interesting flora and fauna. It is a world of tide-related channels and salt meadows surrounded by dunes and dikes. In spring, it is full of birds, including terns and avocets, and becomes a mauve carpet of flowers in summertime.