Minecraft wiki entry for Managing_slimes_in_superflat_mode, associated text: The key idea behind both trap types is to ensure the slimes are trapped rather than killed; once enough slimes are trapped, the mob spawn rate in that area is greatly reduced and new mobs — slime or not — will largely stop spawning. When you want to do some mob farming, simply move 128+ meters away from the slime pit, at which point all the slimes in it will despawn and mobs of all types will then start reappearing elsewhere as they normally would. A good tip is to do this either shortly before nightfall or shortly before dawn to maximize the number of farmable non-slime mobs that appear. By this method you can roughly control the numbers of mobs appearing in an area by luring more or fewer slimes into your slime pits.