I've encountered a problem in the Underworld... I get down to 3,432 feet and the character and monsters disappear. I still move, but go anywhere including into solid ash or lava. No damage or anything and the monsters can't attack me but then they are off in solid ash too. Once in the ash, however, I can't move anywhere... use the Mirror to go back to the spawn point. Happens both in buildings and when digging down. Annoying because there are items down there I can't get to. I submitted a bug report on this, but I am not sure if it isn't just a variation of a bug I had seen already reported.