Where the Semeru dwarves dug too deep, the elves took their soul gems, and Carl decided to add water.
The ninth floor comes up on the trail as history rather than as a destination. Clint Smashgrab's version is an accusation: the High Elves went down there once, back when Scolopendra was still asleep, and that is where they collected the soul gems that power their magic. When the worm began to wake they ran — but not before stealing something else, an item that later shielded the whole royal court from Scolopendra's Nine-Tier Attack.
Carl has heard a different version from the Cookbook: that the Semeru dwarves dug their city too deep, a reverse Tower of Babel. It is that story, he reflects, that gave him the idea of filling the place with water in the first place.
Not visited in this chapter — described only as a city dug deep, home to the Semeru dwarves and, deeper still, to something sleeping.
The deep dwarf city whose plundering is blamed for waking Scolopendra, and the floor Carl chose to flood on the strength of that legend.
Also appears in books you've already read — spoiler-safe to their endings: