Sitting at the crossroads of Europe and Asia, the ancient city of Istanbul has seen thousands of years of trade, battles and invasions. Now it is the scene of one of the most audacious engineering projects in the world.

The Marmaray Rail Tube Tunnel, due to open in 2010, will not only be the deepest underwater tunnel ever constructed. It willalso pass within 16 kilometres of one of the most active geological faults in the world. A major earthquake is not only expected, but imminent. No wonder the Turkish government is calling it the project of the century.

Istanbul is divided by the Bosporus strait that connects the Black Sea to the north of the city with the Sea of Marmara to the south (see Map). Part of the city lies in Europe, on the western side of the strait, while the rest is in Asia. Two road bridges cross the strait and there are plans for a third, but ever since the Ottoman sultan Abdul Mecit suggested it in 1860, city leaders have dreamed of building a tunnel to link the two halves of the city. Last year, a mix of technical expertise, foreign investment and national pride finally came together to make the sultan's dream a reality.

This time, the plan is not so much to unite an empire as to deliver modern Turks from traffic hell. Today, crossing the Bosporus means either a 3-hour trip by rail and ferry, or braving gridlock in narrow, 2000-year-old streets and the two overcrowded road bridges. The Marmaray project, which takes its name from the Sea of Marmara and "ray", the Turkish word for rail, aims to ease the strain by replacing car traffic with an upgraded rail service that will whisk commuters between Europe and Asia.

The plan is first to improve the existing railways on both sides of the strait and then extend them to the coast via tunnels bored through bedrock. The centre section, under the Bosporus, will be a 1.4-kilometre tube made up of several shorter sections that will be built on land, floated into position and sunk into place (see Diagram). End to end, the tunnel will be 12 kilometres long.

It might sound straightforward, but the project engineers face a major geological hurdle. Twenty kilometres south of Istanbul lies the North Anatolian fault (NAF), where the Anatolian plate that underlies Turkey, Greece and the north Aegean is being squeezed to the south and south-west by the surrounding Arabian, Eurasian and African plates. The result is what geologists refer to as a right-lateral strike-slip fault, similar in size and type to the San Andreas fault in California. The NAF runs for 1600 kilometres across northern Turkey, and the abutting plates move about 2 to 3 centimetres relative to each other every year.Shaky ground

Earthquakes along the NAF are common. In the past seven decades Turkey has endured seven quakes of magnitude 7.0 or greater. While some earthquakes release the stress that has built up on a fault, seismologists have come to realise that others simply shift it along the fault, leaving it even more prone to slip. Almost every quake along the NAF in the past 100 years seems to have set up a larger one, to the west. The process appears cyclic: quakes march along the fault in sequence until stress falls below a certain threshold, and then start again after a period of quiet.

In 1997, geologists studying the most recent cycle predicted that the next shock would hit near the port city of Izmit, 80 kilometres east of Istanbul (New Scientist, 28 August 1999, p 5). Sure enough, a major quake of magnitude 7.4 struck close to Izmit in August 1999, followed by another in D