It happened before and could easily happen again which is another reason why some people with the means have been building underground shelters in order to protect themselves from the potent radiation. As the story goes Bill Gates is to be one of them. For a more detailed explanation you may want to go to the suggested web-sites.
“As the story goes Bill Gates is to be one of them.”
Bill Gates has his home (well, main home I suppose) on the shores of Lake Washington near Seattle. Much of it IS built into the hillside. To be more heating efficient, lower profile, etc. so they said. I suppose he may have been thinking of a CME as well.
However, they now think that a large rupture of the Seattle Fault that runs underneath Lake Washington could cause a tsunami in the lake (and Puget Sound). The runnup of water in the lake would inundate Gate’s home as it is fairly low on the hillside.
Well, like Grandma said - “When its your time to go - its your time to go.”