The Tuskegee syphilis experiment, which began in 1932, is seen as one of the darkest moments in medical history, and one of the most evil US government experiments on humans. In the Tuskegee syphilis study, doctors at Tuskegee University were trying to find out more about syphilis. But things went off the rails quickly - researchers lost their funding, they lied to their test subjects (impoverished African American men), and didn't treat them for syphilis even after they knew penicillin could cure the infection.
What began as a plan to provide treatment for men who had no regular access to medical care turned into a 40-year-long unethical syphilis study about what the disease did to the human body. Specifically, what it did to the African-American human body, since, at the time, researchers believed that different races responded to diseases in different ways. In the name of science, doctors watched as men went blind, went crazy, and died of the terrible disease.
It wasn't the first (or the last) time scientists exploited African Americans for research purposes - as is proven with the case of Henrietta Lacks. But the US syphilis study in Tuskegee, AL, is now seen as the most unethical medical study in US history and is considered one of the creepiest government conspiracies of modern memory.