Minecraft wiki entry for Drowned_farming, associated text: At the most basic level, the trap consists of a single water block that the zombie falls through from above, landing on a block (or a hopper) so the zombie's head remains in the water and drowns it. A sign on the wall holds up the bottom of the water block, and solid blocks enclose the water on all sides. The enclosing blocks may be glass to aid your identification of drowned zombies versus non-drowned zombies.