I also lost a good dog in a place I dug regularly and called a puppy spot. Its a hard call to predict what might happen before the terrier goes under, but I guess we all do it, which is fair enough based on past experiences at the place.
I know of other places that look like very easy sandy places, but the holes lie under a layer of sandstone and you need bars to chip through it, it always takes a couple of hours toil, looks like a 5 min job!
Thats it mate,and places out in fields that have been landscaped over by farmer and end up looking like small places but have lots of deep tube