Sand dunes are large hills of sand formed by the wind. They are commonly found in deserts and along coastlines, where loose sand is abundant. The shape and size of sand dunes can vary greatly, depending on factors like wind direction and the amount of sand available. These natural formations play an important role in the ecosystem. They help protect coastal areas from erosion and provide habitats for various plants and animals. Some well-known examples of sand dunes can be found in places like the Sahara Desert and the Great Sand Dunes National Park in Colorado.