Norway’s submerged bridges would be held up by pontoons along the surface, connected with trusses to keep everything stable. There’s also the possibility that the structure may be bolted to the bedrock below to provide added stability. Each bridge system would consist of two tunnels, side-by-side: one for traffic in each direction. Despite the unconventional arrangement, officials say it will be much like driving through an ordinary tunnel for commuters. With 1,150 traffic tunnels already in use throughout the country (35 of which are underwater), Norwegians likely won’t be too confused by the arrangement.
So why not a normal bridge? Unfortunately, the difficult terrain in these regions makes them unsuited for an ordinary bridge: they’re simply too wide and too deep. (And at a mile deep, digging a conventional tunnel is unfeasible.) One alternative to the submerged bridges might be to build a suspension bridge or a floating bridge over the water, however, these designs have the disadvantage of being susceptible to damage from rough weather. They also run the risk of interfering with Navy ships that sometimes use the water for training.
http://inhabitat.com/norway-to-build-worlds-first-floating-underwater-traffic-tunnels/