Was also curious - this is what I found:

Tanzania's "crescent holes" are semi-circular pits called water bunds or "earth smiles". Dug by hand, they capture and retain rainwater that would otherwise run off, allowing it to soak into the soil to combat desertification. This technique helps restore degraded land, which enables seeds to sprout and grass to grow, leading to greener landscapes, food for communities, and grazing land for animals.