However, if the victims of theft are lost to antiquity, the land properly belongs to non-criminals who are in current possession. Another problem with land ownership is that land might never have been used and transformed, making any existing property title to the land invalid. Libertarian theory only validates ownership claims to land that has been transformed by human labor. If land has never been transformed, then no one can legitimately claim ownership. A claim to land that has never been put to use is invalid. The enforcement of such a claim against a first user would be an act of aggression against legitimate property rights. Another issue that arises in land ownership is adverse possession, where an individual assumes land is unowned, works it for a length of time, and the original owner appears and orders the individual's eviction. The common law of adverse possession sets a time span of 20 years, after which the intruder retains absolute ownership of the land, but libertarian theory holds that land needs only to be transformed once by man to pass into private ownership. Therefore, if the land bears the mark of a former human use, it is the individual's responsibility to assume that the land is owned by someone, and any intrusion upon the land without further inquiry is an act of aggression against legitimate property rights. Chapter 11, "Land Monopoly, Past and Present", discusses the issue of land monopoly and its two forms: feudalism and land-engrossing. Feudalism is when landholders continue to aggress against peasants who work on the land, while land-engrossing is when arbitrary claims to virgin land are used to prevent first-transformers from accessing the land. Rothbard argues that both forms of land monopoly clash with the libertarian principle of non-ownership of land except by actual transformers. Land monopoly is a widespread problem in undeveloped countries, especially in Asia, the Middle East, and Latin America. However, the United States is one of the few countries that is virtually free from feudalism. American conservatives tend to overlook the land problem in undeveloped countries and only focus on promoting free market and private foreign investment. However, foreign investment based on land monopoly and aggression against peasants is similar to feudalism and must be dealt with in the same way. Rothbard provides examples of land aggression and monopoly in the modern world, including the Shah of Iran owning more than half