The Heart of the Abyss is what the three components of the World Below Nehrahn are referred to.  There are technically three components to this patch of caverns: the Heart of the Abyss, the Ring of Fire, and the Watershed Breach, each wrapped around the others like the layers of an onion.  Much like the surface, however, most just know the whole of it as the Heart of the Abyss.  The Na'a'Pel Tribe of Abyssal Demons dwell in these caverns.