One of the repercussions of the fall of man was the start of nuclear decay. This in turn is the beginning of hell, since the explosive rate of decay, concentrated in the core of the Earth, caused the core to melt.
Graves and Pits
March 20th
If you're on a downward course and leave the lower mantle, once you enter the great gulf, there's nothing to stop your fall into the core. This means that the pit refers to everything below the mantle.