In Gene Wolfe's The Claw of the Conciliator, part two of The Book of the New Sun, the protagonist Severian is tricked into entering a network of subterranean mines (where he encounters the "man-apes", possibly zoanthropes?).
Here, Severian somehow wakes a giant being, which he never sees, just hears:
If an ogre were to eat at the very legs of the world, the grinding of his teeth would make just such a noise. The bed of the stream (where I still stood) trembled under me, and the water, which had been so clear, received a fine burden of silt, so that it looked as though a ribbon of smoke wound through it. From far below I heard a step that might have been the walking of a tower on the Final Day, when it is said all the cities of Urth will stride forth to meet the dawn of the New Sun.
And then another.
I don't think we ever learn explicitly what this creature was (not uncommon with Gene Wolfe), but Severian seems to know it at the time he's writing down his memoirs. Just a couple of paragraphs below:
A fourth step sounded and I turned and fled, groping before me with the blade. What creature it was we had called from the roots of the continent I think I now know. But I did not know then, and I did not know whether it was the roaring of the man-apes, or the light of the Claw, or some other cause that had waked it. I only knew that there was something far beneath us before which the man-apes, with all the terror of their appearance and their numbers, scattered like sparks before a wind.
What is this creature, and when does Severian learn what (he thinks) it is?
The only other reference I can find to "walking towers" is this much later one, in the fourth part, The Citadel of the Autarch:
Far behind it loomed a machine that flashed fire, a machine that was like a tower walking.
This seems to be a war machine, as it is encountered during the war against the Ascians. You wouldn't think the thing beneath the Saltus mines was a war machine, but the connection is made even more explicit only a page or so later:
I opened my mouth to cry for help, then closed it again, thinking I might call upon myself something more terrible than that I had once waked in the mine of the man-apes.
There are also some more references to the continents of Urth, and the myth that "all the cities of Urth will stride forth to meet the dawn of the New Sun", that perhaps tie into "the roots of the continent" from which the creature was called. Dr Talos's play, for example, says that new continents are ready to rise with the coming of the New Sun (phrasing from Gene Wolfe's appendix to The Citadel of the Autarch). It's also said that Abaia, another large creature that lives in the sea, "will one day devour the continents" (The Shadow of the Torturer); very Lovecraftian.
By googling this question, I've not found any definitive answers, but it seems strange to me that this mystery shouldn't have one. I have, however, found references to the man-apes being the servants of
the Autarch; although I can't remember learning that from reading the book, it fits with the man-apes standing down in awe of the Claw, or probably actually of the Conciliator. I assume this means that the creature under the mines is not under the Autarch's control, since the man-apes fear it and it awakens when the man-apes stand down, but other things Severian seems reluctant to describe in detail seem to be parts of the Autarch's mysterious defenses. Does this mean that the creature in the mines is a servant of Abaia?