Pics are here: https://imgur.com/gallery/R1FyV

I recently purchased a house with really nice oak flooring. Unfortunately, the previous owners weren't really good at cleaning up after their dog, and there are urine spots which soaked through their area rugs and stained the flooring underneath.

I'm in the process of refinishing the floors, and did passes with 60, 80 and 100 grit which took out all of the paint drips and other marks/scratches but, the stain remains. So, I hit the spot with a belt sander using 36 grit, then a pad sander 80 then 100. I've gone pretty deep into the wood and can still see the stain (last pic) and am afraid of going any deeper as the polyurethane will pool on the finish coats.

What should I do? Wood bleach? Keep sanding? Help!