An underwater bridge is a military tactic that was employed during World War II and the Korean War.
Underwater bridges are typically constructed of logs, sand and dirt just beneath the surface of the water in a river or similar narrow body of water. The bridge allows heavier vehicles to cross the river driving through only shallow water, and they are difficult to spot from the air and if spotted are difficult to knock out with air strikes as the water protects the bridge from blasts.
The concept was used by Soviet Union troops during World War II. North Korean troops also used the tactic during the Korean War, particularly to cross the Naktong River during the Battle of Pusan Perimeter.[1]