In Ohio, you can acquire legal title to another's real property if you prove exclusive possession and open, notorious, continuous, and adverse use for a period of 21 years. But, the general rule is that you cannot adversely possess property against the state.  So, when a friend here in Ohio emailed me this very interesting question, my first thought was "sure... I think so, if all of the elements are met, why not?"  But, the Fifth Amendment to the U.S. Constitution provides that private property shall not be taken "without just compensation." Does that rule out the possibility that the government can take title by adverse possession -- without going through the process of a condemnation proceeding and paying just compensation? Hmmm... it really is an interesting question.