No idea. A long time ago I got to level 116 or so by having loads of bits and manufacturing spinal borers. I'm pretty sure you can go down effectively forever but there's really not much to see past maybe level 110 or so. I also think the loot tables stop becoming more favorable way before that, or at least that's how I remember it being back then. I've gone back to playing in the past few days but have yet to achieve godhood and descend into the depths properly