An ancient attempt at building a functional biological creature superior to humans, now known as the Naive Human Machine.
Heavy reliance on such principles as "bigger is better" and "break things and apologize later" let to rushed decisions. Technology at the time was clearly not ready and desperate quickfixes were implemented left and right.
If you look closely you will see that parts of this giant machine made from bones are operated by simple hand cranked levers and ropes and the brains inside the head are composed of smaller human brains. I got partially inspired to draw this thing from a horror short story I read years ago about two towns battling each other using a kind of giant made from their own bodies. I can't remember how it was called, I'm afraid, but I'm pretty sure it was written by Clive Barker, the guy who wrote Hellraiser books.
And I am planning to use this in a personal game project as a small level. You will be able to explore the hollow bones as you would a regular cave.