In the early 20th Century Lewis Hine documented child labor and newly arriving immigrants at Ellis Island. He was extremely prolific, producing iconic photos of children in textile mills, coal mine, and other shocking places. Most of the work is in the public domain. Both the Library of Congress and the New York Public Library have large collections of his work.
In my humble opinion, Mark's work is in many ways the 21st Century version of that work. Hopefully, someday it will make a similar difference.