I went exploring far and wide without having jumped down the "Great Hole to Ix". I was in Baby Steppes and moved into "Groddle Ladder" and all of a sudden the Rock congratulated me for Completing the "Go Jump In a Hole" quest - even though I'd done no such thing!
I figured out that I must have taken the long way around to get to the same place that the Hole usually takes people to. Perhaps the logic for completing this quest should be based on the act of passing thru the hole rather than just on a player's arrival in the area of Groddle Ladder.