Flooring Tile - Where/what is the subfloor under the tile?

The floor tile (1in by 1in) is sagging and cracking at the grout seams in the center of the bathroom. The sag covers about a 1-foot by 2-foot section of the floor. About 40 years ago my uncle, the tilesetter, put a new tile floor over the existing tile floor. I believe the original tile floor was from circa 1930 but I don’t know the condition of the floor when it was tiled over.

I removed some of the newer tile around the crack/sag and a crack in the “subfloor” was exposed not necessarily following any joist. I was hoping that a wooden subfloor would be exposed when the two layers of tile were removed. Instead it looks like the tile from 1930 was installed as a “mud job” with a few inches of concrete below the floor tile. I don’t see any wood subfloor and I don’t want to dig around too much without knowing what I am getting into and because there is a very nice plaster ceiling right below the bath.

With all this, even the portion of the floor that sagged appears pretty solid. I was thinking since it’s a small area I could just fill in the cracks I see in the subfloor, level it off and re-install the top level of tile. This is more of a quickie repair but I would love to avoid replacing the entire floor.

Any suggestions would be appreciated. I would really like to know more about the existing subfloor/original construction as it would affect what I eventually do. It has me puzzled. Thanks in advance for any help.

I called another uncle who was a tile setter and asked him about what is under my bathroom tile and used for a subfloor. He said they used to pour concrete between the rafters and actually make a concrete pad. He also said they stopped doing it right before or right after WW2 when he was apprenticing. He said how they prevented from getting on the ceiling below it but didn't follow all the details. What he described looks an awful lot like what I saw. He also mentioned that there was a lot of weight in that floor and its a bear to get out. Another interesting comment was they sometimes used coal ashes in the cement as opposed to sand because the ash was lighter.

I still know the integrity of the subfloor but I think I will go for the patch job. It seems stable and replacement sounds like a major job. So much for old time construction.