This is all excellent advice, really appreciated.
So far it sounds like the best route it to dig down and try to figure out what I'm dealing with.
This morning I took a fence post, placed it into the hole, and drove it ~2ft before I hit solid rock, so right now I'm thinking it might be a rock seam.Also I think 'Coal country' is further east of me, and I have no other sinkholes on the property.