T. Toennesen was a railway engineer. In 1900 he journeyed with Dr Georg Hartmann* of the South West Africa Company through the Kaokoveld. Later, in 1909, he undertook an expedition to the Kaokoveld for the South West Africa Company, accompanied by Werner A.A. Hahn*. A series of 18 Reports of the Kaokoveld Expediton (1909), written by Hahn*, are housed in the National Archives of Namibia. After the territory became a protectorate of the Union of South Africa in 1915, Toennesen published a paper on "The South West Africa Protectorate" (Geographical Journal, 1917, Vol. 49, pp. 282-300, with map) in which he made many references to the climate of the territory and provided rainfall figures for 25 stations. The Toennesen Mountains in the Kaokoveld were named after him.