 There are NHLs in seventeen of the twenty-one counties in the state.Mercer County has fourteen NHLs, in and around Princeton, New Jersey.National Historic Sites, National Historic Parks, National Memorials, and certain other areas listed in the National Park system are historic landmarks of national importance that are highly protected already, often before the inauguration of the NHL program in 1960, and are often not also named NHLs "per se".