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Echo dunes \EK-oh DOONS\ n. Steep dunes that form upwind of an object such as a fence or building. In windy, sandy landscapes, echo dunes can block or bury man-made structures. Using a wind tunnel that generates gusts up to 78 miles per hour, researchers in China are trying to better understand how echo dunes accumulate by looking at what happens to moving air when it encounters an immovable object. The team found that when wind wallops a slope of 60 degrees or more, the air bounces back, creating a vortex. This reversal, they report in the Dec. 9 Journal of Geophysical Research, could shape sand into echo dunes and may provide insight into how to protect desert architecture.