In what sense can someone come to 'own' a piece of land?

First of all, the question of what grounds private ownership applies to all goods, not just land. So I'm going to treat the question as the more general one about how one can come to own anything. There are two basic approaches to this question: a "natural rights" approach and a "social institution" approach. On the former approach, people have rights to own goods in the same sense in which they have other basic rights, such a right not to have their bodily integrity violated by others. The question for such an approach is how one acquires a property right in any particular good, which amounts to the right to exclusive use of the good and a correlative duty on the part of others to not interfere with the owner's use of the good. The most common answer to this question derives from Locke, and it divides the question into two: how does an unowned good come to be owned by someone, and how can ownership of an already-owned good be rightfully transferred. The answer to the second question is fairly...