Can this floor be salvaged?

[mabloodhound]

Nails should ALWAYS be set before sanding a floor. And filler added to the holes and any other imperfections.
Remaining thickness is hard to determine unless you can get to an edge or cutout and actually measure the boards. It isn't the power of the sanders, but the tongue and groove flooring that determines how much you can sand.
Once you get down to the groove, if you go too far, the top edge of the groove will break off and/or expose the tongue underneath. Not a desirable look and only way to fix this is to add a new board(s).

 

[floridayankee]

Very good - I forgot all about the tongue and groove. But today a different thought hit me - Why do we see all those nails ? I recall that tongue and groove
are nailed in such a way that the nail gets concealed by the next board. If all else fails then you have no choice but to nail from above. So why does this floor have lines of nails ?
I figure years later you might need to tighten a squeeky floor, and you'd hope you have access from below, so you could nail or screw from under. Maybe the ceiling is finished below ??? Or is this a floor that was never tongue and groove ?