What is that down there?, was mine and it got rejected. It seems basically the same.
Well, but maybe there's nothing there in fact? You assumed there's some kind of 'that'. That would be "Co to tam jest na dole" or similar.
Okay, but that seems to contradict what guliwer521 says, "We say "na dole" if we refer to something already being there. " I'm not sure if I understand.
Well, my way of thinking was like that: your friend gets to the bottom of a well and you yell to him "What's down there?" - and perhaps the well is completely empty and his answer is "Nothing, absolutely nothing!".
But perhaps you see that there is something down there, but can't see what it is - so you ask "What is that down there?" and 'that' refers to that unknown thing that you do see.
That's how I interpret it and that's why I offered an a bit different translation.
We say "na dole" if we refer to something already being there. "Na dół" is a description of a direction - going down, looking down etc.