The UC Davis Beagle Experiments—and all laboratory experiments on dogs, in general.

For more than 50 years, UC Davis bred and performed radiation experiments on tens of thousands of beagles. I read that they often surgically removed their vocal chords prior to the experiments to reduce the ambient noise level.

Obviously this pales in comparison to a lot of human tragedies, but the sheer scale and the fact that it turned into an EPA Superfund site without too many people knowing about it is shocking.

https://www.google.com/amp/s/amp.sacbee.com/news/local/education/article219515130.html

https://theintercept.com/2018/05/17/inside-the-barbaric-u-s-industry-of-dog-experimentation/