Minecraft wiki entry for Spawner_traps, associated text: On the 25-24 block levels, dig a 1×2 hole for seven blocks in the direction the mob spawner. On the seventh block in, dig one block downwards. Dig in the forward direction for eight more blocks (again, 1×2). You should notice the water block placed on the 24th level should stop where the path drops by one block. Place a water block against the wall of the drop. Notice how this water block stops 1 block short of the path.