This article is about the military structure. For the undersea rail tunnel, see Channel Tunnel.
An underwater bridge is a military structure that was employed during World War II and the Korean War.
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 seen, air strikes have difficulty in destroying them as the water insulates the structure from blasts.