I remember reading a list of industrial disasters, and Dam breaks always had absurdly high death tolls. I remember reading one from China that had several hundred thousand IIRC.

I couldn't find the exact Wikipedia article, but this one is still a pretty interesting read.

https://www.wikipedia.org/wiki/List_of_accidents_and_disaste...

Edit: I think it was this one: https://en.m.wikipedia.org/wiki/1975_Banqiao_Dam_failure

Though, of course, the exact numbers are contested.