No clue. They don't even say in the sequel (and if they did I'd call bullshit because the sequel isn't canon) I've always assumed that he was some sort of serial killer who dug his way out of prison, and was driven insane by the amount of time he spent underground digging his way out of prison, so when he finally did get out he wanted to nothing but keep digging
Except Rourke. That guy was a dick.
If you're going to insult something at least make an attempt to spell everything right