Minecraft wiki entry for Construction, associated text: Casting means one fills layer by layer with lava source blocks and pours water over them, turning it into obsidian. It is very difficult as one needs a diamond pickaxe for mistakes and large amounts of buckets for effective transport of water and/or lava, not to mention the high danger of working with lava. For a large amount of lava needed, it is advisable to either find one or more lava lakes or to go to the Nether. (more to come)