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Considering that this basin of ours is encircled by mountains, and mountains keep leaning toward their angle of repose even without any help from earthquakes, it’s not surprising that gold makes repeated appearances.

The result, says Gilchrist, is cohesion that generates a negative angle of repose due to a negative coefficient of friction.

"They didn't exist. But to understand how these grains are flowing uphill, we calculated what the stresses are that cause them to move in that direction. If you have a negative angle of repose, then you must have cohesion to give a negative coefficient of friction. These granular flow equations were never derived to consider these things, but after calculating it, what came out is an apparent coefficient of friction that is negative."

Mr. Deshpande and his colleagues set up their sand pyramids to be below the angle of repose, meaning that theoretically they should have just sat there.

Materials like sand have what is called a natural angle of repose — if the sides of a pile become steeper than a certain angle, its grains will sweep downward in miniature landslides.