Dirt under the tub
I found the same thing. My wife decided to re-caulk the tub and in doing so did not create a good seal. We eventually noticed a wet spot on the outside of the house just outside of the bathroom. I took out the surround, replaced the drywall, dried everything out, put in a new surround and sealed everything up. After a while I noticed the wet spot was not going away. Then there was a musty smell in the bedroom next to the bathroom. I pulled up the carpet in the corner and the pad underneath was all wet. Once I pulled the drywall away from the corner I saw why it wasn't drying out. There was dirt inside the area under the tub. I stopped by my local home depot to ask if this was usual practice and the guy I spoke with said it is possible they may have used dirt to hold the tub in place as the older ones did not have any way of attaching to the framing.
However, when you look at the dirt, it's only on one side and it's piled up in a way that it would have been impossible to set it in that amount of dirt. It was almost to the top of the tub both front and back. I also recall finding dirt in the wall cavity behind the toilet when I replaced the wall covering there too.
My initial thought was it was something done by the original owner. His son sold us the house and there was a pretty good crack in the foundation. He mentioned that his dad tried to do a few fixes himself but didn't know exactly what. My guess was that the dirt was something he did but I didn't know how it would apply and I still don't.
I live in La Mesa just outside the San Diego city limits. From what I can tell the tub is original and my house was built in the mid 1950's.
If anyone else has had the same situation it would be good to hear from you and get more insight.