The abyss is the last unknown area in hell, having its dark depths which no one knows where the bottom of it is. Some speculate it leads to the center of hell but for the most part no one really knows how long or far down this great thing goes. but they know traversing it leads to something completely different and not quiet but very no scratch that extremely odd. Also within the abyss is many many many different brantching tunnels and levels. and then there's the Sin Fall's. which is a massive waterfall of molten souls which pours them into the abyss. the only one to have fully explored this to day is auro. and it took him.. 13 years.