At the very bottom of a closed desert basin is a floor so flat it looks poured: a pale, cracked, barren plain where nothing grows, sometimes crusted white with salt. This is a playa, a dry lakebed, and the reason it is dead level is that it is the lowest point for miles, the place everything eventually runs to and nothing runs out of.
That last part is the whole mechanism. A playa forms in a basin that has no river draining it — water flows in off the surrounding slopes but can only leave one way, by evaporating. So runoff and sediment collect at the bottom, the water sits as a shallow sheet after a hard rain, and then the sun takes it back into the air, leaving behind the very finest clay it was carrying and every dissolved salt it could not lift. Do that season after season and the floor becomes a dead-flat pan of tight clay, sometimes crusted with the salts the evaporation concentrated.
It is really the end of the bajada. Everything the mountains shed, water sorts down the sloping apron of the bajada, coarse rock near the range and finer and finer material downslope, and the playa is where the sorting finishes: the last, finest particles, dropped where the flood finally dies. Read the whole basin as one system and the playa is its drain.
The surface tells its own weather in cracks and crust. After a rain it is a sheet of standing water or sticky, bottomless mud; as it dries the clay shrinks and splits into a mosaic of polygons; salts wick up and whiten it. The same ground can be lake, mud, and hardpan within a single season, which is why it grows nothing: too salty, too airless, too often drowned or baked.
It matters even to those who never stand on it, because a playa is a dust machine. The fine clay it hoards is exactly the light material the wind lifts, so a dry playa feeds the region's blowing silt, the same dust that stacks under a desert pavement and darkens into a haboob wall. This ground is not the playa itself; the homestead sits up on the sloping bajada, not down on the basin floor. The playa is the low place the whole slope drains toward, off in the distance.
If you ever cross one, watch the sky. Wet playa clay is slick mud over hard pan, and a dry lake floods fast.