This is a railway transport project in Istanbul, which consists of construction of a railway tunnel along the bottom of Bosphorus strait and modernization and upgrading of already existing urban railways. It is building with the purpose to connect the two halves of Istanbul across the Sea of Marmara, from Halkali on the European shore to Gebze at the Asian coast. There is also provision of 440 new vehicles for the underground passenger traffic, delivered by Hyundai. 

The name of Marmaray comes from combination of Sea of Marmara, which lies southward, and ray, which in Turkish means rail. 

The project includes building of 13.6km tunnel, crossing the Bosphorus strait and renovation of 63km urban railways. The tunnel at the bottom of the sea will be protected from earthquakes by a 1.4 km log tube. The tube consists of eleven parts, 8 are 135m long, two are 98.5m long and the last element has length of 110m. In order to place the tube properly, in the bottom was dug a channel with depth of 4.6m. When finished, the facility will allow transportation of 75 000 passengers per hour in both directions. 

The building of Marmaray started in 2004 and the tunnel was finished on September 23th, 2008. Overall completion of the project was expected to be in 2009, but it was delayed several times. At present it is expected to be finished in October, 2013. 

via :wikipedia.org