It might refer to the one hell where children who die young before their parents are forced to stack rocks for all eternity, only to have their efforts destroyed by an oni every time they make progress.
How are we going to cross the Sanzu?Mn...Ohhh!Look, look!As long as we were here, I tried stacking some rocks!Very impressive!Half a day later.I guess we wait patiently until it comes for us...Meanwhile...The boat comin' or what?BadummWell, actually, there's supposed to be a ferry...Sanzu RiverThree hours later.Welll?Oh!That's bad luck, so cut it out.Come to think of it, where is Koishi-dono?