The First wonder, Speaking Sands For nearly a century, man has been baffled by the sound of singing sand dunes. The songs they emit are almost as diverse as the countless theories about how they occur. The sound is produced when the sand on the surface of dunes avalanches. It was once thought that these sounds were produced by the friction between the grains. More recent studies have revealed that the sound continues after the sand has stopped moving and the song that the dunes sing varies depending on the time of year. Some researchers now theorise that the sound is caused by the reverberation between dry sand at the surface and a band of wet sand within the dune, hence it changes seasonally. There are approximately thirty locations around the world where these booming dunes can be heard; the earliest records seem to date to Marco Polo’s time in the Gobi Desert. However you don’t need to adventure among the dunes to hear them sing; the strange sound, said to be like the drone of a low flying propeller plane, has reportedly been heard up to ten kilometres away from its source.