  From fairy circles to patterned ground and columnar joints, natural patterns
spontaneously appear in many complex geophysical settings. Here, we investigate
the origins of polygonally patterned crusts of salt playa and salt pans. These
beautifully regular features, approximately a meter in diameter, are found
worldwide and are fundamentally important to the transport of salt and dust in
arid regions. We show that they are consistent with the surface expression of
buoyancy-driven convection in the porous soil beneath a salt crust. By
combining quantitative results from direct field observations, analogue
experiments and numerical simulations, we further determine the conditions
under which salt polygons should form, as well as how their characteristic size
emerges.
