…what could cause a mushroom cloud?
Lots of things. How about a wood fire?
About 22-23 years ago, I was working evening shift at Brown’s Ferry. Going in one day, I stopped at the convenience store about 4 miles away. The employees asked me what had happened at the plant. I said I was headed in and didn’t know yet. They replied that they’d seen a mushroom cloud coming from that way.
Turns out it was this. Basically a huge but mostly hollow box of wood, it went up so fast it produced the mushroom effect.