Here's an Atlas Obscura entry about the mass grave of an estimated 6,000 Confederate soldiers detained by the Union as prisoners of war in Chicago: https://www.atlasobscura.com/places/confederate-mound
Even the number of people buried there is not known with certainty. And there seems to have been quite a few petitions to remove the monument with the 4,200 known names entirely. [1]
[1]: https://huskiecommons.lib.niu.edu/cgi/viewcontent.cgi?articl...