Minecraft wiki entry for Obsidian_farming, associated text: When mining in this way, the top layer of obsidian usually has lava underneath it. Not only is this hazardous, it can also destroy the mined blocks before they are collected. A solution is to place water next to the obsidian and stand in it. When you mine an obsidian block adjacent to the water, the water should flow and freeze the lava before it has a chance to burn anything. It is also possible to place the water above the obsidian, however this is more likely to get in the way and decrease mining speed. Be careful to periodically move the water, especially if you are standing in it. Otherwise, it is possible that the water will reach its limit of 7 blocks and push you or items into the lava.