The enormous ribcage of some skeletal dragon or something. There's big buildings built up in the shadow of its bones and a major industry just carving out chunks of ancient bone to use as building material. No one's ever even heard of a living creature that huge... some folks think it must have been the god of dragons or something. It's been there as long as the recorded history of the city... some of the ribs have been broken off entirely and used up by the locals. But much of the bone is unusable due to quality or awkward placement.