Minecraft wiki entry for Wither_cage, associated text: The wither can be safely spawned and allowed to float up. This way, the obsidian is never in range to be broken when the wither is damaged. This Nether design also has the benefit that if the wither escapes through the bedrock, it would end up above the bedrock ceiling. Unless the player built something above the bedrock, that wither will not cause problems, although it will likely never be seen again.