I thought you were referring to the usual divot in the dirt at the end of the slide. The playground/park outside of my old condo had some sort of rubberized surface.
The weird thing about the pic, it looks like there's another opening behind that red post, and maybe another near the shade way behind the slide. Makes me wonder if they were supposed to be filled with something, which the edges of the concrete around the openings would still seem dangerous. Or maybe, "They all float down there".