Originally Posted by
joypulv
In the US? You know of land that is unowned by either a private party, corporation, town, city, county, state, or the feds? I'd be surprised. Have you looked at the deed maps in your town hall?
As for adverse possession of owned but unused land (which is probably what you mean), your fence would have to be 'openly hostile' for the minimum number of years in your state (many are 20).
I have seen brick walls installed one day and torn down by the rightful landowner the next. As long as he is confident that it's his land, he doesn't have to mess around suing anybody. The burden is on the tresspasser/encroacher.