Underwater bridges, typically constructed of logs, sand and dirt just beneath the surface of the water in a river or similar narrow body of water, allow heavier vehicles to cross the river driving through only shallow water. Air reconnaissance finds such "bridges" difficult to spot; even if spotted, air strikes have difficulty in destroying them, as the water protects the structure from blasts.
Soviet troops used the concept during the Khalkhin Gol campaign (1939) and in the Great Patriotic War (1941-1945). North Korean troops also used such structures during the Korean War (1950-1953), particularly to cross the Naktong River during the Battle of Pusan Perimeter of August-September 1950. In the Vietnam War the Ho Chi Minh trail used such bridges.