Here's a shot to give you an idea of where this sink is.
This is the spot, on one end of the sinkhole that I was concentrating on. I had stair stepped the floor down to here. I had thought any channel would have went that way, but I opened up a hole that lead back under my stair steps, and it seemed pretty significant.
I went to work dismantalling the lowest stair, and found that the floor of is was made of pretty large rock, that was suspended above another floor a couple feet below. I tried to be careful taking it apart, but about halfway through, the whole floor colapsed right in front of me. From there, I could see that the lowest level of bedrock that you can see in the picture, has a horizontal void under it for the entire length (at least a foot hight), and it is void under the bedrock on the opposite side of the hole that I started on. I don't know how far back the void goes, but it's farther than I can reach my arm. Plenty of cool air too. The void has bed of loose rock. It almost looks like a dry river bed, alot of the loose rock down there is perfectly polished. Now, I have to dismantle my upper stair, because the void goes underneath it, and that leaves me way the heck down there! Do caverns start underneath bedrock like this?
(crappy picture, because the debris from the floor colapsing covers most of the void entry)