Minecraft wiki entry for Slime_farming, associated text: Before building a slime farm, first decide whether to build it in a slime chunk or swamp biome. Every chunk that is generated in a world has a 10% chance of being a slime chunk. Within slime chunks, slimes can spawn when Y < 40. This method requires an underground room to be dug out before the farm is constructed. Slime spawning in swamps can occur between Y=51 and Y=69. This method has the advantage of not requiring a dug-out room prior to building the farm, and the size of the farm is not limited, unlike the slime-chunk method, although the spawn rates are lower in a swamp.