Hey all, thanks for the replies and comments so far, I've cleared out as much as I'm willing to at the moment, I've started hitting mainly clay and stone and I'm getting a fair amount of water in the clay (is it possible this dirt is actually attached to outside dirt?)

Anyway, 2 pipes visible in front of the clean-out opening, one large pipe visible out the back side, both seem to go right into the wall of the concrete lower lip

At this point my thought is backfill with clean gravel and install a thick foam board as others have suggested, possibly with concrete poured on top once I'm comfortable that will be ok, maintaining access to the clean-out somehow

But any advice is welcome