its a naturally occurring substance and it does regenerate, but everyone uses a little bit unconsciously sll the time. when populations were low great wizards could learn to use it intentionally. As population increased power decreased. it left the cities first, the far uninhabited mountains still have dribbles not in use. When the aliens attacked, smart people ran for the hills, the weird ones who still half believed in magic. they noticed the effects pretty quickly as the population declined
