http://copyright.cornell.edu/resources/publicdomain.cfm
Copyrights usually do not involve pieces of paper in strongboxes because everything that is created is copyrighted by default--even the unpublished material. There is a reasonable chance people could inherit copyrights and not realize it.
You can't technically (legally) publish something that is under copyright--but the risk of it being a problem is nonexistent in some cases. To have a problem, some entity's interests must conflict with your own. If no such entity exists then a problem cannot exist.
I feel very bad for content creators that have fallen victim to the trap of copyright. All it takes for them to truly die is a lack of fame and a family that doesn't know about or care about the works. What are the chances that seventy years from now a physical copy of that person's book will find its way into the hands of somebody that cares enough to save it?
Thank the gods for ones and zeroes! Everything
we say will be around as long as there are humans, aliens, or Caesars to read it.
