The article presents the results of research on a uniquely preserved linear box-shaped furnace found on the Kuyahtanar iron smelting site in the Altai Mountains (Russian Altai). Furnaces of this type were the largest in North Eurasia, yet reliable data on the structure and emergence thereof have not been available for a long time. The article provides a detailed analysis of the structure of the furnace as well as 3D models of its different parts.