During the rainy season, some kind of worm emerges from the soil in the savannah and creates large mud casings, which then dry to varying degrees of hardness. Some break down when you walk on them, some don’t. And the ground is littered with them, to the point that it is virtually impossible to put a foot down on anything approaching a flat surface. On ground which has been recently burned, you can see these as you walk on them, and can try to walk on game paths, were the game has broken down many of the casings. But where the grass has not been burnt, there is no avoiding these things.
Here's what most, if not substantially all, of the ground looks like:
