From Death Valley to Chile to Iran, similarly sized polygons of salt form in playas all over the world — and subterranean fluid flows might be the key to solving the long-standing puzzle of why.

[Above] Mars: Summertime view of polygons, highlighted by dark, windblown sand, on the floor of a crater at 71.2°S, 282.6°W. [Above] Earth: Polygonal cracking can be seen on the floor of Alatna Valley ...