Plot hole: Didn't Dexter ever notice the giant fire pit in the ant farm?
Suggested correction: This is more of a question than a mistake. (To be fair, I don't think questions were a feature yet back when this was posted.) No, he didn't. He only noticed the fire pit (and the colosseum) when he was inside the ant farm. It most likely just wasn't visible to him from the outside, or it was too small for him to see it.
Plot hole: I thought the monster was in Japan. How did it end up in America at the end?