"During the rainy season (December to March) the salt flat gets covered with a layer of water. As the water evaporates under largely still conditions, the salt forms hexagonal shapes (optimal for heat transfer) on its surface known as Bénard cells, an example of the Rayleigh–Bénard convection [1] phenomena." [2]
Image: "Hexagonal formations on the surface of the Salar de Uyuni (Bolivia, 3,656 meters above sea level) [0] as a result of salt crystallization from evaporating water": https://en.wikipedia.org/wiki/Salar_de_Uyuni#/media/File:Sal...
"During the rainy season (December to March) the salt flat gets covered with a layer of water. As the water evaporates under largely still conditions, the salt forms hexagonal shapes (optimal for heat transfer) on its surface known as Bénard cells, an example of the Rayleigh–Bénard convection [1] phenomena." [2]
[0] https://en.wikipedia.org/wiki/Salar_de_Uyuni
[1] https://en.wikipedia.org/wiki/Rayleigh%E2%80%93B%C3%A9nard_c...
[2] https://commons.wikimedia.org/wiki/File:20170809_Bolivia_158...