Some structures built at the end of the second millennium stand out on account of their length or complexity. Without claiming to be exhaustive, here are a few examples:

The Seikan tunnel

In 1988, when the Simplon tunnel was still the longest railway tunnel in the world, the Japanese finished digging the Seikan, between Honshu and Tokaido islands. It is 53.9km in length and runs 240 metres below the sea across a 23.3km strait. The soil was too hetereogeneous and too little was known about its composition for tunnelling machines to be used, so the tunnel was excavated in the traditional manner with explosives. Many problems had to be resolved during construction.

The Channel Tunnel

This was constructed shortly after the Seikan tunnel, which superficially it resembles as it is 50.5km long and runs under the sea. However, the technology involved was very different. It was excavated with a tunnelling machine as it passes through homogeneous chalk whose characteristics were well lnown. This made it possible to achieve rapid rates of advance - up to 51 metres in a day. It runs only 30 metres below the sea.

The Tokyo Bay Tunnel

Safety systems sometimes take up a considerable amount of space, as in the case of the submarine tunnel under the Tokyo Bay Baie de Tokyo.

In addition to these railway tunnels, we can mention some road or railway tunnels crossing other sea straits or mountain chains that are either planned or in the course of construction:

• In Denmark, the Størbelt, which links the islands of Seeland and Fionie.
• The Øresund Tunnel between Denmark and Sweden,
• In Switzerland, the new 57km Saint Gothard railway tunnel, which is under construction.

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