Every few weeks strange, unseaworthy giants weighing 55,000 tonnes are floated out of a specially-built dockyard in Denmark, and gently towed on a one-way journey to their destination in the sea-bed near Copenhagen.
This bizarre ballet has being going on since last August. The "vessels" consist of enormous reinforced concrete tubes, 176 metres long and 42 metres wide, sealed at each end so they float. Joined together in a trench in the sea-bed, they will form a roadand-rail tunnel which is part of a link between Denmark and Sweden.
The Oresund link, as it is called, will provide a road-and-rail connection from the Danish capital, Copenhagen, to Sweden's third largest city, Malmo. The people of each city can see each other across the narrow channel and up to 3,000 commuters make the hour-long ferry crossing every day. When it opens in the middle of the year 2000, the new link should change the way these close neighbours relate to each other.
Building a 10-mile bridge would have been cheaper, but this would have posed a danger to aircraft using Copenhagen airport. So the first two miles of the link are by tunnel, which costs about twice as much per kilometre as a bridge. This leads to an artificial island that is being built up behind sea walls with material dredged from the sea-bed, and is about 2 1/2 miles long. The third element is a five-mile bridge with an elevated section so ships can pass underneath.
The main contractor for the tunnel is a consortium of five companies from Sweden, Denmark, France, Britain and the Netherlands. It accounts for nearly £400 million of the total cost of £1.5 billion.
A British firm of engineering consultants also involved, the Symonds Group, is basing its proposal for a tunnel under the Irish Sea on the same concept. Building sections of tunnel on land and burying them in the sea-bed is cheaper than the more traditional method of boring through bedrock.
The process begins in a special dock in Copenhagen, where the tunnel sections are cast in reinforced concrete. In the case of the Oresund link, there are five separate tubes in each segment: two for a railway line in each direction, two for a two-lane roadway in each direction, and an escape and service tube between the two road compartments.
The section is cast in stages, gradually emerging from the casting hall until it has reached its full length of 176 metres. Ballast tanks are placed inside, and steel bulkheads are fitted at either end so it is watertight. Then it is moved down a slipway into a flooded lock. The floating section is then towed to a trench dredged in the sea-bed.
Satellite navigation with an accuracy measured in centimetres allows it to be positioned precisely. The ballast tanks are adjusted so that the section can sink, and it is lowered into position from two floating pontoons. A cable between the previous segment and the new ones draws them together, and a rubber joint on each face provides a watertight seal. The last stage in this phase is to pump out the small amount of water trapped between the temporary bulkheads of the new segment and its predecessor. When this happens, the water pressure from the sea forces the two segments together.
The bulkhead can then be removed, and thousands of tons of concrete are poured into the bottom of the new segments. This is to keep it from floating to the surface again, and only then can the water be pumped out of the ballast tanks which have been keeping it on the sea-bed.
When the operation is completed, the segment which weighed 55,000 tonnes on land will weigh 200 tonnes on the seabed, as it is still basically a hollow concrete box. The trench is backfilled and the top covered with rock. The top of the tunnel segment is generally below the level of the sea-bed.
Some of the other technical obstacles which have had to be overcome include different rail operating systems; Swedish trains travel on the left and Danish on the right. They also use different voltage and signalling systems. The last six months before the link opens will be devoted to testing the various systems.
The consortium providing the link is jointly owned by the states of Sweden and Denmark. Nearly all of the £1.5 billion cost of the project is being met by loans raised on the international market. With a state guarantee from not just one but two countries, the Oresund Consortium says it has been able to obtain lower interest rates than either government could have done.
It expects that traffic tolls will pay back the cost of building it over about 30 years. The EU is providing less than 5 per cent of the cost.
Road users will subsidise the rail link, and the crossing is also paying for upgrading the road and rail links on land which lead to it.
The project has had its share of opposition: environmentalists' biggest concern was that the artificial island would restrict the movement of water, leading to a change in the salinity or oxygen content of the Baltic Sea. The shape, size and position of the island has since been altered to meet concerns.
Opponents of the project outnumbered its supporters at one stage by more than two to one. The consortium says the latest poll shows they are now evenly balanced, at 43 per cent each.
When the Oresund link opens mid-2000, the jobs market, property prices and the economies of the two cities could all be affected. But it will be a return to the past in some senses: Malmo was once part of the Danish kingdom, until the Swedes recovered much of their territory in the 17th century. The route may also restore some of the trading links which flourished between the two regions before rising sea levels flooded the land between them thousands of years ago.