The frontier walls built by different dynasties have multiple courses. Collectively, they stretch from Dandong in the east to Lop Lake in the west, from present-day Sino-Russian border in the north to Qinghai in the south; along an arc that roughly delineates the edge of Mongolian steppe. A comprehensive archaeological survey, using advanced technologies, has concluded that the walls built by the Ming dynasty measure 8,850km (5,500mi).[4] This is made up of 6,259km (3,889mi) sections of actual wall, 359km (223mi) of trenches and 2,232km (1,387mi) of natural defensive barriers such as hills and rivers.[4] Another archaeological survey found that the entire wall with all of its branches measures out to be 21,196km (13,171mi).[5] Today, the Great Wall is generally recognized as one of the most impressive architectural feats in history.[6]