I obviously wasn't that clear - there is no imminent danger of collapse but two things are happenning. One is that the voids above the tunnels are perhaps making their way gradually towards the surface at a very slow pace. If this is happening archaeology is redeposited out of context in a lower position and perhaps broken up and lost - and although this is only a small area it could in very simplistic terms lead to a trench appearing on the surface. The second thing that is happenning is that material underneath Silbury has been damaged by air and water and will be lost unless they go in and get it.