Bridges go over water. Tunnels go under water. How about the Archimedes bridge, a hypothetical tunnel design that goes through water instead?
If you want to get a road or track across a body of water, the most obvious solutions are to build a bridge, pontoon, or causeway over it, or dig a tunnel underneath it. These work for shallow waters, but as soon as you run into deeper straits both options become difficult. What if you wanted a bridge or tunnel between Europe and North America, for example?
Science fiction writers, as always, were ahead of the game. Jules Verne’s son Michael wrote about the possibility in 1888, as did Arthur C. Clarke in 1946 and 1956. German author Bernhard Kellermann wrote a book called Der Tunnel which was popular enough to be made into a film four times between 1915 and 1935. In reality, alas, a tunnel underneath or a bridge over the Atlantic is just too expensive and logistically challenging.
There’s another option. Instead of going over the ocean or under the ocean, it’s possible to create a tunnel that goes through the ocean instead. This is the Archimedes bridge, also known as a submerged floating tunnel.
Build pieces of tunnel on dry land and then sink them into the water. The tunnel is designed so that it stays more or less suspended at the desired depth. In technical terms, the tunnel gets close to hydrostatic equilibrium: like a submarine, the weight pushing the tunnel down is close to the buoyancy it gets by displacing water. All it takes to keep the tunnel in place is a set of cables, either attached to the sea floor or coming down from the surface.
The Archimedes bridge is theoretically viable, and there have been several patents, plans, and proposals from Norway, Italy, China, Japan and elsewhere. Now we just have to build one and see if it would actually work.