For example, about six months ago when I read Assata, the autobiography of Assata Shakur and one of the best books I've ever read, I was startled to learn that the 13th Amendment to the Constitution explicitly allows slavery in one case:
Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.Which made a lot of things make sense that hadn't before. Or more accurately, fully explained things that I hadn't realized didn't make sense. Like, before I learned about that, I thought the existence of private prisons was the entire justification for all the increased incarceration of the past few decades, despite the fact that private prisons actually account for only a small percentage of the totality of the American prison landscape, and despite the fact that I didn't have a clear concept of how incarceration led to profit. I just hadn't thought it through.
Now I know. Prisoners are slave laborers. Not metaphorically, literally.
(An aside: right now I'm finding it useful to think of the 13th Amendment not as abolishing slavery so much as changing it from a private system to a government one. The mixed public-private incarceration system that we're seeing now is essentially a symptom of the accelerating melding of the public and the private.)
And a related thing I just learned about in Are Prisons Obsolete? is the "convict lease" program, instituted in the wake of the 13th Amendment, in which, as the name of the program indicates, convicts could be leased for labor. The especially neat thing about this is that after the end of slavery, the Black prison population in formerly slave areas shot up dramatically. After all, imprisonment is the punishment of the individual by taking away his or her rights; under slavery, Black people had no rights, so their punishment took other forms. Now that Black people at least theoretically had rights, they could be taken away--and not only that, but the lawmakers set about making all kinds of things illegal specifically so that Black people could be "duly convicted" of them.
So you've got these suddenly primarily Black prisoners available for "lease." You purchase the right to use them for a month for some fee. As Angela Davis points out, this often led to worse treatment than under slavery, because when you own a slave you have an investment in them, and don't want to work them to death because then you have to go buy another one. With leasing, there's no reason not to work them to death, because you can just go lease another one, which you would have done anyway.
According to wikipedia, the "system was slowly phased out in the early 20th century, with Alabama being the last state to outlaw the practice in 1928." Which is awfully recent for something so completely horrific, so intensely brutal, to have happened and been entirely forgotten.
And of course there is still a vast, and exponentially increasing, sea of Black (and other shade) bodies in prison being used for profit to this day.