In the middle of a huge plain dominated by the Langberg Mountains rise curious dunes, protected by this 3 300ha reserve. Between 30 and 60m high, these yellow, brown and white dunes are redolent of mystery. When the temperature and humidity are right, they make an intriguing whistling sound, a phenomenon known as "Reynolds dilatancy", observed in detail by French scientists in Morocco. Some seem to yell rather than whistle!