The Steading of the Hill Giant Chief: The Lower Caverns

From the introduction:

Background: When originally published, G1: Steading of the Hill Giant Chief made several references to a series of caverns and other features below the southern and western portion of the Dungeon Level that was published in the adventure. Those caverns and dungeons were never detailed, and the only clues to their nature were in the descriptions for some of the encounter areas (18, 19, 21, and 23, in the original module).
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n the Gygaxian spirit of putting some of the most interesting discoveries in the most unlikely of places, the following addendum is offered for the Dungeon Master who would like to spice up those lower caverns beyond nothing more than fungus, carrion crawlers, and lizards.