I have seen one of those plugged Spanish holes once, but only was a photo in a prospecting magazine. Did you ever see Jill's Donkey Prospector articles in GPAA magazine? Well, she helped her friend Texas Jack (think it was his name) by hauling planks and rebar up the mountains to his cache site in NM. Keeping rock rubble from filling their hole using rebar/planks. A flat rock slab covering was removed except the hole looked empty. So, Jack got his pendulum out, which he kept saying, "dang thing still says there is treasure down in there" kind of mumbling. Finally, Jack went down with headlamp, hammer, pry bar gear. Jack comes back up smiling holding what appears to be a crude smelted Spanish bar. In the photo it looked like gold, no assay had been done yet at this point (excited Jack published the story first).