Minecraft wiki entry for From_Below_CSG, associated text: This design is in fact a midsize building, 9×7×7 or so, with the generator proper on the roof. You mine from within, facing a current held back by slabs. As with any building, unless you're in Peaceful difficulty or Creative mode, you'll want to secure it against monsters, perhaps by fencing or glassing in the sides. (This will also help in case of a major lava spill.) Note that once you've got it built, the pillars become optional, though you will want roof access.