The Great Wall (Wanli Changcheng) is one of mankind's greatest achievements and dates back more than two millennia, when a series of shorter walls were systematically linked during the Qin dynasty (221-206 BCE) to protect Chinas northern borders. The Wall, originally built to keep out barbarian invaders from the north, stretches more than 5,900km (3,700 miles) from the Bohai Gulf in the Yellow Sea to Jiayuguan in the mountains of Gansu province. While the bulk of the wall is now in ruins, some sections have been repaired in recent years. The Ming rulers, who in 1368 put an end to the alien Mongol Yuan dynasty, forcing them back to their ancestral homelands outside the wall, decided to rebuild the Wall into a formidable barrier, the 'Ten Thousand Li Great Wall'. This enormous project took over 100 years, but in the end failed to prevent the invasion of China by the Manchus in the mid-17th century. As a result, much of the wall in northeast China today dates back to the Ming period.