If a dog is too large, he will not be able to get past turns in the tunnel, and will have to be dug to every few feet. If a dog has to dig to the quarry when the tunnel tightens down (as it invariably will), the dog will have to push dirt behind it to progress, which can result in the dog being “bottled” by dirt from behind. In such a situation the dog will have a very difficult time getting out on its own if it cannot turn around.