Recent video coverage of a side door on the body near the paw was found to descend to a pool of water several stories down. I think it was on one of the Egyptology shows with that young Egyptian archaeologist on Ancient Aliens.
I don’t think the pool has been plumbed yet.
There's a New Kingdom-era shaft somewhere between the Sphinx and the river (if memory serves) that was found and excavated in the last ten years or so, turned out to be a shrine to Osiris, something like that. Has to be kept pumped out because of the water table. Campbell's tomb (shown on the line drawing up there) is a New Kingdom-era shaft up[ behind the Sphinx (w of the enclosure) and that doesn't "probably serve as light and air shaft to subterranean passages" as the caption says, it's been excavated (by someone named Campbell) right to the bottom, it contained a number of NK burials.