Underground iron farm

An iron farm made of four pods, each holding three villagers and one zombie trapped in a boat. The panicking villagers spawn golems into a pit below, where water carries them to lava and hoppers feed the chests. It uses no redstone and cheap materials. Java only.

• Java only. Bedrock spawns iron golems by different rules and several people finished the whole build before finding that out.
• Let the villagers sleep a full night before you scare them, so the game registers the village. This is the most common reason nothing spawns.
• Spawnproof the ground all around the pit with slabs, buttons or path blocks. Roughly 17 blocks out is quoted, and any cave you dug through on the way down counts too.
• One zombie per boat. Two zombies interfere with the villagers' sleep and production stops. The villagers also have to actually see the zombie.
• Fill the two blocks above the boat once the zombie is in. A golem can spawn there, kill the zombie, then suffocate or despawn.
• On 26.2, leave a block of space around the beds so the villagers can still move, otherwise it does not work.