Four European countries are meeting in Turkey to sign a five-nation agreement for the long-planned 3,300km Nabucco natural gas pipeline. Once completed, the line will bring up to 31 billion cubic metres of gas a year from the Caspian and the Middle East across Turkey and into Europe. It will give an important alternative energy supply to Russia, which already meets 30% of Europe’s gas needs. But much still remains to be agreed on, not least where the gas will come from.