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The Ranzhuang Underground Tunnel冉庄地道战遗址, located in Qingyuan County 清苑县 in the southwest of Baoding, is a tunnel system with a total length of 15 km, stretching along four main and 24 sidetrack tunnels built in the war against Japan in 1942. Entries to the system were hidden, either inside houses or down a well. It was used for surprise attacks across the battlefield with hidden gun holes and was an important element of guerilla warfare as propagated by Mao Zedong in his 1937 publication On Guerrilla Warfare (论游击战) where he emphasized the significance of flexible military tactics vis-à-vis an enemy with larger resources and manpower. The tunnel became famous on a nation-wide level after being depicted in the 1965 film Tunnel War 地道战 (for the film see here) directed by Ren Xudong 任旭东 (born 1924) and produced by the August First Film Studio. A remake of this film directed by Li Xiaoting 李小亭 was produced at that site in 2009, aired in 2011 under the same title as a television series with 40 instalments. After 1949, the tunnel site became a key heritage site protected at the national level (全国重点文物保护单位), serving the purpose of promoting patriotism and national defense education as a red site.