We've undertaken the refinishing of an old tongue and groove floor. Part of it had been sanded and bare for years, so the wood was quite grey there. We were quite surprised to find that our sanding revealed very noticeable dark brown stains. Some neighbors thought it might be oil from an old printing press that had once lived in the room. Others declared they were certainly water stains. Well, we all agree they're pretty ugly. Splotchy, long, deep they are. The worst area shows a distinct outline of something big.

So... Even though we hate the stains, we're wondering if trying to sand them out is too risky. More background: 1.There appears to be no subfloor, just "joists" I guess thgey're called. (It's an old army infirmary.) 2. The floor has been sanded pretty aggressively in the past. 3. We are hitting nails in some spots and seem to be all the way through one of the sides of the "groove" in a few.

Gosh maybe I answered my own question of "Can we risk sanding much more?"
But those stains are soooo ugly! We dont want to go dark. Any brilliant suggestions to my dumb question?
(Forgive me. This is my first time.)

Maybe consider staining the floor to hide the stains.

An old printing press in your house? thats a cool story, seriously I think I would leave the stain on the floor and theme the room around it. make the stain a conversation piece.

I recently bought a home with water damaged buckled floors. Believe it or not I was able to sand the damaged areas, bleach the black stains out of the grain apply a coat of poly and then screened the entire floor and blended it perfectly.

So my point is, if bleach worked for my mildew type stains, perhaps you should look into a stripper of some sort that may address your issue before you keep sanding. Using the right stripper may actually cause the oil to rise to the surface and allow you to clean it up and then bleach the wood to original color and allow you to finish in a natural finish or light stain.

Anyways I hope maybe I opened a research door for you, good luck!