The Imperial Sand Dunes are the continent's largest active erg, or large area covered by sand, stretching for more than 40 miles in a band averaging 5 miles wide. Sometimes called a sand sea, they are believed to have formed from the windblown sands of Ancient Lake Cahuilla and continue to creep 12 inches to southeast every year. Prevailing northwesterly winds drive the dunes onward, with smaller dunes moving faster than larger ones. This dynamic landscape includes hills almost 300 feet high and constantly shifting terrain, making it a popular destination for off-highway vehicle enthusiasts and giving rise to terms such as slipface, razorback, and witch's eye.