The Great Wall of China is the largest and the longest wall to have ever built by human. Several walls were being built from as early as the 7^th century BC by ancient Chinese states: selective stretches were later joined together by Qin Shi Huang (220-206 BC), the first Emperor of China. Little of the Qin wall remains. Later on, many successive dynasties have built and maintained stretches of border walls. The most well-known sections of the wall were built by the Ming Dynasty (1368-1644).

Collectively, the Great Wall stretches from Dandong in the east to Lop Lake in the west. The construction materials were mainly earth, wood, stones, sand, bricks, used depending on construction era and construction site with different climate and local materials. Its average height ranges form 6 to 7 meters and the tallest part reaches around 14 meters. A comprehensive archaelogical survey, using advanced technologies, has concluded that the walls built by the Ming dynasty measure 8.850 km. This is made up of 6.259 km sections of actual wall, 359 km of trenches and 2.232 km of natural defensive barriers such as hills and rivers. The entire wall with all of its branches measures out to be 21.196 km.

"..., using advanced technologies, ...." (Par. 2). The word "advanced" is synonymous with ____