Descending into darkness behind and below the Boudoir, the Undercaverns are much more natural in appearence than most of the Firecavern complex. They are murky and dark, only now and then lit by odd bioluminescent mosses or mushrooms, and have the same kinds of pits and holes and tight squeezes and dead ends that come with natural caverns.
It has significantly less of the fiery nature of the rest of the caverns. In fact, a single small stream, starting in a modest spring at the very back of the Boudoir, wends its way down and through the Undercaverns. It passes through deep pools here and there, becomes mere trickles elsewhere, and now and then something or other can be found living in these pools and streams. The Undercaverns are hushed, and reach far and deep, ending in the Abzu where no one is meant to go.