At the Dungeon of Zork entrance to Hades, the River of Wailing, which is a branch of the Styx, flows on one hand, and on the other the River of Terrible Fire. These two rivers unite around a bald and barren mount of rock, and the dreadful waters then hurl themselves with a great thunder into the Great River of Acheron. The spur of land jutting into their tributary is the furthest point (roughly a half mile within) that one may tread before being required to cross the Acheron into Hades. It is this pinnacle where blood libations must be offered, as well as where the Hades Shuttle Service arrives to ferry the recent dead into the Underworld proper.

SOURCE(S): The Zork Chronicles