Due to its isolation and geopolitical situation, the Tibesti Mountains were long unexplored by scientists. The German Gustav Nachtigal, dispatched by Otto von Bismarck, was the first European to explore, albeit with great difficulty, the Tibesti in 1869. Prior to Nachtigal's expedition, an American along with two missionaries had attempted to enter the Tibesti range, but were killed by a Toubou warrior. While Nachtigal provided an accurate description of the population, his account discouraged any new adventure into the Tibesti for over 40 years, as he was convicted by the Toubou of spying for Bismarck and only released after the intervention of Maï Arami Tetimi. Later expeditions carried out between 1920 and 1970 yielded valuable information on the geology and petrology of the range. The French anthropologist Charles le Cœur and his wife Margaret, a geographer, lived among the Teda of Tibesti between 1933 and 1935. Le Cœur was the first to closely study the Tibesti populace, but the outbreak of World War II prevented him from publishing his research. French Colonel Jean Chapelle published a book on the Toubou and their lifestyle in 1957. In 1965, the Free University of Berlin opened a geomorphological research station in Bardaï; however, research at the station was interrupted several times due to the Chadian Civil War, and the station was ultimately closed in 1974.