A permanent tunnel should not ever be contemplated. Perhaps if people would go and look (theres a good picture in Burl's Prehistoric Avebury) at the Marlborough/Merlin Mound they would understand what a terrible disfigurement doors would do to the hill. That mound is in a terrible state, the tree cover does it a service. The idea that Silbury and its surrounding landscape should be turned into a peepshow like Stonehenge does'nt bear thinking about.