BEIJING, May 11 (Xinhua) -- The most extensive preservation project for the "Great Wall" has been launched by the Inner Mongolia Autonomous Region.

    "A survey of Ming dynasty (1368-1662 AD) Great Wall is in full swing, and repair has started for Qin Dynasty (221-206 BC) Great Wall," said Mingri, deputy director of the regional cultural department.

    Inner Mongolia has the longest section of the Great Wall among all provinces. The Wall built in different historical periods runs more than 20,000 kilometers in Inner Mongolia, Mingri said.

    The project focuses on protecting the original architecture, including some reinforcement work. It will take two to three years, he said. The major work this year is to finish repairs on the Qin-era sections of the Wall and investigate what needs to be done on the Han Dynasty sections, he said.

    The cost is estimated at 100 million yuan (about 14.3 million U.S. dollars), of which more than 5 million yuan has already been invested, according to Mingri.

    The Great Wall, mainly located in the central and western partsof the region, features architecture from several eras, ranging from the Warring States Period (403-221 BC) to the Ming Dynasty, he said. Some sections, built in the Qin Dynasty, the Han Dynasty (202 BC-220 AD) and the Ming Dynasty, are listed as national cultural relics.

    "Most of the Great Wall has been altered beyond recognition by 2,000 years of history, which makes repairs a most urgent task," said archaeologist Wang Dafang.

    The wall, unique in size and style in the world, was China's line of defense during much of its long history. Different sections meander across thousands of miles, passing through many provinces and autonomous regions, including Gansu, Ningxia, Inner Mongolia, Shaanxi, Shanxi, Hebei and Liaoning in north China.

    Its construction began during the Warring States Period, when sections were built in scattered strategic areas.

    The most visually striking section is generally considered to be a well-preserved 6,350 km section from the Ming Dynasty.