Fech fech

Fech fech (Arabic: فش فش‎‎) is a very fine powder caused by the erosion of clay-limestone terrain. This pulverized soil is common under a thin crust in deserts. It is not determinable from the surface and can therefore pose a significant transportation hazard acting as a surprise "trap" as the ground collapses beneath a vehicle miring it in a quicksand-like substance.

Fech fech is common in the Qattara Depression, making that portion of the Sahara Desert impassable by most vehicles.