I asked them and they said it was for a sump. I'm really considering buying it. I don't have room to upgrade just yet I don't think. But it probably wouldn't be a bad plan to start collecting stuff to do so. I mean heck I've only had my 29 for 5 days. Haha. I wonder if there is a reliable way to plug the holes on it. I don't know if I want to attempt a sump anytime soon