The Kelso Sand Dunes cover 45 square miles and tower over 700 feet (200 meters) high. The dunes were formed mainly with sand from the Mojave River and are between 10,000-20,000 years in the making. The piling up is caused by a confluence of shifting winds in this area. They are considered "booming dunes" - when the sand is dry and you cause a small "avalanche," a low booming sound can be heard.