I think this is really exciting news. The Smithsonian has released over 2.8 million images into the public domain under Creative Commons Zero (CCo) license.

Location of material: https://www.si.edu/openaccess

License description: https://creativecommons.org/publicdomain/zero/1.0/

Original article on the release: https://www.smithsonianmag.com/smithsonian-institution/smithsonian-releases-28-million-images-public-domain-180974263/

Edit: fixed link