Eradication of knowledge often happens unintentionally. If we stroll through the city and encounter an impressive piece of world, we tend to look it up on the internet. Nice, if we find some good information. But sometimes, you get confound: the only online presence of the place you just discovered is the governmental approval for the budget of demolition. If somebody had known more about this place at the moment of initiating such a decision, would this person have acted differently? To produce common knowledge - facts, publicly accessible - are a premise for a collective negotiation of what is worth to keep.