I personally believe that any un-homesteaded property is fair game for anyone to homestead and claim it as their own as their own.

A good book that describes this is "For a New Liberty" by Murray M. Rothbard

Also listen to Stephen Kinsella's podcast "Kinsella on Liberty" he gives very good explanations.

If the property is currently homesteaded then a mutual agreement between the property owner and the individual wishing to take possession must be arranged.

I don't believe governments can do this as a government is not a person and therefore has no ability to possess anything.