Minecraft wiki entry for Drowned_farming, associated text: Go down into your collection room. As zombies fall into the water-trap channel, they wander back and forth across the tops of the hoppers with their heads in the water channel. After 30 seconds of having its head submerged, a zombie starts to drown, quivering visibly for 15 seconds, and then changes color. Its pants turn from blue to greenish. Its eyes, visible through the glass blocks, also change to a blue glow.