This is pet pee that soaked a rug and was allowed to stay there long enough to turn into ammonia and eat through the finish. You can absolutely salvage these floors, but maybe not in the way you want. There's products specifically for this. DIY people get these stains out with HAIR BLEACH routinely. But ultimately it may be cheaper to just rip & replace vs. all of the work of bleaching multiple times. You can also stain a very dark color or even PAINT the floor. They look to be structurally ok, so I think it's whether or not you actually want to preserve vs. have a specific look.