Archaeological sites are an irreplaceable, nonrenewable resource. Once destroyed, they are lost forever. Many cultural activities modify the earth's surface, destroying sites. City, road, and reservoir construction and deep plowing in the past century have drastically accelerated destruction. Air pollution decays materials which have survived millennia and commercial art markets inspire pothunters and grave robbers to loot countless sites. Since prehistoric people tended to live in the same favoured locations as modern settlements, the need to preserve archaeological sites too often is lost to progress.