Saharan Star Dunes

A little while back, I posted one of the greatest examples of the concave-convex illusion I've ever seen, where sand dunes in the Sahara look like little craters when flipped upside-down. At the time, I was more interested in the illusion than the dunes themselves, but that all changed when the NASA Earth Observatory posted a stunningly lovely satellite picture of the same kind of dunes, called star dunes: Seriously, right? Dunes are formed when winds pick up and move sand grains. The shapes of ...