Oh good Lord. Honestly, as a claustrophobic myself, the scariest part of this whole thing was his first dream. Being trapped in a mountain like that, unable to move....guh. And when he described his second dream, that he was forced to go into the hole as punishment for a crime, my thought was, "They could just go ahead and kill me; I'd rather be executed than have to walk into a hole like that."
Of course, the end is pretty creepy too, but not in the same way. In fact, after that first dream, the ending is almost reassuring to me. "Phew, thank God, it's just some kind of hideous monster. That I can deal with. Trapped under the mountain? Not so much."