In 1941 geological surveys were conducted on Tsushima Island and test bores were sunk to 600 metres (2,000 ft) close to Kyūshū.[17] By August 1942, Japan's South Manchurian Railway Company had created plans for an 8,000 km (5,000 mi) rail network stretching from Manchukuo to Singapore.[2] Against this backdrop, Japan took its first concrete step for a fixed link to and through Korea to connect it with its planned vast rail network in Asia, with the construction of several bridges as well as the completion of its 3.6 km (2.2 mi) Kanmon undersea railway tunnel joining the Japanese islands of Kyushu and Honshu.