MPN11,
I was worrying about how the "bearing strength" of the former open land around the former "hole" would be affected by wholesale building on top. There have been cases up here in the past where alarming chasms have suddenly opened over long closed mine workings below. The data and its keeepers stored in a former 'hole' might be secure - for all eternity !
Retrieving it might be a problem though !
Danny.