I dig and dig but can't move the large rocks. We will have to use explosives.

But how much? We only have limited supplies of gun cotton. If we use too little then we just waste it, as the rocks stay in place. But if we use too much then we could cause another cave in and be back where we started.

So do we err on the side of caution, and use just a little? Or do we err on the side of boldness, and use a little more? Or do something else entirely?