No idea, but presumably that would be after we've explored a fair bit of it. We've barely scratched the surface.
It seems quite plausible that there could be ground water. If that's the case (and I suppose even if it's not) it wouldn't be at all surprising for the cave systems to contain microbial life.
That isn't even remotely what I said though. Indeed I'm in favor of that in the end. But you're trying to invoke "just one more" when realistically we haven't even started for real yet. So it seems more like you just want to entirely skip the sterile part of exploration where we're careful not to cross contaminate. Which is certainly a valid position (though I disagree with it) but I think you should be up front about that.
It seems quite plausible that there could be ground water. If that's the case (and I suppose even if it's not) it wouldn't be at all surprising for the cave systems to contain microbial life.