The tunnels were the most difficult and time-consuming part of construction. Of the twenty tunnels, machines were only used for the Nåli and Vatnahalsen Tunnels—the rest were built by hand. Hand-construction was done by drilling up to through the rock, filling the holes with dynamite and blasting. Construction of tunnels started in 1924 and the first tunnel was completed in 1926; the last tunnel was completed in 1935. On average, tunneling proceeded at between 116 and 180 man-hours per meter (35–55 hr/ft). The work caused death or lifelong respiratory problems through silicosis caused by inhaling the smoke. There were two fatal accidents, one in 1925 and one 1938, both related to tunnel work.