Minecraft wiki entry for Obsidian_farming, associated text: If you have obtained a large number of lava buckets, it may be more efficient to "cast" the lava rather than drenching and mining it. In this process, the lava is placed as desired and then covered in water to convert it into obsidian. In this way, obsidian structures such as Nether portals can be created in place without the use of a diamond pickaxe.