Indeed. I was going to comment on that very structure.
The Osireion has been studied for decades attempting to ascertain the source of its water system (from which does NOT originate from bedrock, as the subsurface is merely sand) and there are no answers.
The revelations about the Osireion’s true subterranean structures are fairly recent and remain unexplored, such is the nature of permissions from the reigning king of ‘egyptology’, hawaas himself.
“We have as yet no certain indications of the date of the construction; but the style, the size of the materials, the complete absence of all ornamentation, all indicate very great antiquity. Up to the present time what is called the temple of the Sphinx at Gizeh has always been considered one of the most ancient edifices of Egypt. It is contemporaneous with the pyramid of Chefren. The reservoir of Abydos being of a similar composition, but of much larger materials, is of a still more archaic character, and I would not be surprised if this were the most ancient architectural structure in Egypt.”
Edouard Naville 1914
https://isida-project.org/egypt_april_2018/osirion_en.htm
My, how much we have learned in 111 years. /s
Note: Great pics of the excavation of the Osireion at prior link.
https://evol-forum.ch/giza-en/the-underground-complexes/
The Underground Complexes in Egypt
It is official stated that in Giza and Egypt there are no underground structures or complexes except for tombs. This remains the official line despite the discovery and proof of the existence of subterranean complexes, which certainly are not tombs and some of which are also found outside Giza.
Although it has been proven that megalithic multiple storey underground complexes exist in Abydos the ”Osireion“ and in Saqqara the ”Serapeum” and the ”Persian” Shaft (which is also called the 8th world wonder).
There are also two and three-storey structures cut from stone in the system of chambers beneath the “Mykerinos” Pyramid and especially the “Osiris” Complex. And the Chambers beneath the Sphinx.
Used throughout were huge stone blocks and sarcophagi made of red granite and black basalt and all these structures are unmarked with no inscriptions.
Below you will find descriptions, graphics and photos to better understand these unique complexes.