Bhopal. It injured a lot of people and the series of mistakes that caused it to occur is insane. Edit: this was a chemical leak that killed 2,500 people in the immediate aftermath and thousands more long term.
558,125 injuries were recorded due to it.
Triangle Shirtwaist Fire in 1911
(I feel like this is being lost in time, yet it has more historical importance than other disasters listed here: TSF is the basis of modern workplace safety, union organizing and more.)
BP Oil Spill
I recommend watching the ESPN 30 for 30 documentary on the Hillsborough stadium disaster.