Whose graves are set in the sides of the pit, and her company is round about her grave: all of them slain, fallen by the sword, which caused terror in the land of the living.
( Ezekiel 32:23 )
If you are on a downward course and leave the lower mantle, once you enter the great gulf there is nothing to stop your fall into the core. This means that the pit refers to everything below the mantle.
Principia Mattymatica 