TheGreatWallistheonlyman-madestructureontheearthvisiblefromthemoon.ItwasbuiltduringtheWarringStatesperiod(476—221B.C.)topreventinvasionsbythenomadicnationsfromthenorth.ItwascompletedbythefirstQinemperorwhounitedChinaintoonecountry.ThefirstQinemperormobilizedseveralhundredthousandbuilderstolinksectionsofthepreviouswalls,thuscompletingwhatisknowntodayastheGreatWallofChina.Thewallis6,700kilometerslongfromShanhaikuan,whichoverlooksthegulfofBoHai,toJiayuguanPassinGansuProvince.Theaverageheightofthewallis7.8meters,anditswidthaverages6.5metersatthebaseand5.8metersatthetop.Rectangularlookoutpostswerebuiltaboutevery100meters.

　　ThesectionoftheGreatWallfrequentedbytouristsisinBadaling,75kilometerstotheNorthofBeijing.