Shower Drain... What exactly do I have here?

Doing a full bath remodel, and tore out the old, tired 1979 fiberglass shower stall and found an odd-ball drain assembly, that I was hoping someone could help me out with... There is a 1' x 1' square hole cut in the concrete foundation, that drops in to an 18" deep plastic-lined box (think it's PVC). The box is completely surrounded by additional concrete, that looks to be a part of the foundation. The drain pipe assembly (PVC), including the elbow is fully enclosed in the box, and exits one side to the main sewer line (so I have access to the pipe). Additionally, the plastic liner box is filled with about 4" of dirt (and various other scraps and trash that the original builders decided to throw in there). Anybody have any idea what this is? The closest thing I could find on the web was a "French Drain", but those are primarily used in basements and seem to be a bit different than what I have.

On to my concern... I've purchased a tile-ready shower liner, and when I drop it in place, I can feel slight compression movement in it at the location of the 1' x 1' hole when I put weight on it. Any movement of the liner will cause the grout to crack in my tile floor, and I obviously don't want that. I tried creating a brace with a 2x4 that I wedged into the hole, and covered in RedGuard, but there's still movement. My next thought was to try just filling the "hole" with some form of cement, and levelling it to the rest of the floor. Would this work? Any other suggestions?

Thanks a lot!