Minecraft wiki entry for Defeating_the_wither, associated text: There are many places in the Overworld, but some places are better than others for the wither fight. You can either fight underground or on the surface. It is best to fight underground, because on the surface, the wither can easily fly away from you and dodge your attacks, and would also attack other mobs nearby. It would also blow up huge craters in the surface, or may even destroy your buildings. Therefore, it is best to fight underground, so that it is in a confined space. However, the biome in which the cave is in may make a difference. If the cave is in the mountains, there may be infested blocks hidden among the rocks. If these are blown up by the wither, they would release the silverfish trapped inside, which would also attack you. Therefore, don't pick a cave in the mountains. Make sure the cave isn't too far away from your base, in case you die, but also don't spawn it to close to your base because there is a good chance it will get destroyed.