What I find sad is that some of this work wouldn’t even be necessary if copyright didn’t keep getting extended.
Orphan works are works that are in-copyright but do not have a contactable copyright holder. They’re tricky and annoying as far as reuse goes because while technically they’re not re-usable without permission, how do you get permission? People have discussed this problem at length, but The University of Michigan’s Copyright office — the people who are working on the copyright review management system — are trying to do something about it.
Also mentioned by Roy Tennant, HathiTrust Tackles Orphan Works.