This kind of tunnel would have close contact with nearby houses. It could do double duty by providing electricity, water, etc. It must be sturdy enough to withstand the weight of heavy vehicles on the street above. The second type of tunnel is drilled through the ground at a greater depth, corresponding to the level where subway trains usually go. This technique was used under the English Channel.
Arrows indicate rotational direction.
A third, quite exciting option, is to use floating tunnels . They are lighter than the surrounding water, as they are filled with air. But they are kept suspended beneath the surface by wires, that anchor the tunnel elements firmly to the bottom. This technique is better than floating bridges, since, if they are anchored deeep enough beneath the water surface, the tunnels are invulnerable to waves and ice. The floating tunnel uses technique developed during construction of oil drilling platforms for the North Sea.