Minecraft wiki entry for Drowned_farming, associated text: (If you make the center hole 3×3, you can float a stone block above the hole with a turtle egg on it to attract the drowned to the hole, with trap doors around to make the drowned think there's a path to the egg. The turtle egg doesn't hatch when not on sand. However, getting a turtle egg requires a tool enchanted with Silk Touch, which may not be readily available to a player in survival mode. The farm still works without the turtle egg although the drowned are slower about falling through.)