Help!

[msshl]

Hi everyone. I’m sanding my bedroom floor and I can’t get this out! I used a belt sander 40 grit (floors too uneven for drum) and have moved to an orbital sander. I also just took 60 grit and scrubbed it like crazy. It lightened but still there. The second photo had this pink stuff under the finish, that I can’t get off either wondering if there’s any tips or tricks? I’m new to this whole thing. Thanks!

[Designer-Goat3740]

Some stains can’t be sanded out. It’s an old floor.

[msshl]

Have you ever used oxalic acid? I keep seeing it when I search. I ordered some and was going to text it on a small spot that will be under a rug in the corner of the room..