"Silbury's rubbish - just full of holes and on the verge of collapsing anyway..."

Stupid boy! Those holes are a unique archaeological record of 18th century mining techniques and must be preserved in situ at all costs. Fortunately, our research suggests they are robust structures and we are working to ensure they don't get filled with rubble. We are considering allowing the hill to fall down around them in order to avoid this danger.

What would you have us do? Fill them, so they were lost to posterity!?
Tsk.

(PS - We told our plan to our invisible professor friend and he sent us a letter in invisible ink saying "Hooray!" )