On the Korean Peninsula the underground war reached a massive scale. From experience in the Second World War the US relied upon aviation. North Korean forces suffered heavy losses from air strikes which obliged them to construct underground shelters. Initially underground fortifications were built independently by individual units and their placement was chaotic. Subsequently underground fortifications were united into a single system. The extent of the underground war is attested to by the following facts. The length of the front was while the length of tunnels was . So for every kilometre of front there were two kilometres of tunnels. A total of of rock were extracted.