 It's the melting of the arctic ice, as the climate warms, that makes it possible - and you can understand why they're all piling in. In July 2008, the US Geological Survey released the first-ever publicly available estimate of the oil locked in the earth north of the arctic Circle.  It was 90bn barrels, representing an estimated 13 per cent of the world's undiscovered oil resources. If you're an oil company, or an oil-hungry economy, that's more than enough to make your mouth water.  But wait. Less than a year later, the geologists involved in the programme, known as Cara, the Circum-arctic Resource appraisal, had radically revised their estimate - upwards. Now - in June 2009 - they said the arctic might in fact hold as much as 160bn barrels, which would amount to more than 35 years of US oil imports, or five years of total global oil consumption, and be worth, at current prices, more than $18 trillion dollars (?11.17trn). Forget mouthwatering. Think drooling.  No matter that the polar regions are the most inhospitable parts of the whole globe. and no matter, either, that the arctic constitutes the world's most untouched ecosystem. The oil industry's motto has always been "Can Do", and in the arctic, it's already doing.  Cairn Energy, an Edinburgh oil exploration company founded by the former Scotland rugby player Sir Bill Gammell, was the first in: it is now in the process of drilling four test wells in Baffin Bay, off the west coast of Greenland (it began last year with three wells, none of which struck oil).  Next year Cairn will be followed into the high north by Shell: the anglo-Dutch giant has already spent more than $2bn on seabed leases and hopes to start a massive programme of oil exploration in July 2012, with up to 10 wells in the Beaufort and Chukchi seas off the north coast of alaska, a region that, according to US Geological Survey estimates, holds 25bn barrels of oil. Shell will be followed in turn by the biggest of all the oil "supermajors", and the world's largest company - ExxonMobil.  Last week it was announced Exxon had formed an arctic exploration partnership worth $3.2bn with Rosneft, the Russian state oil group, to look for oil on the other side of the arctic, in the Kara Sea off the coast of Siberia. In doing so, Exxon was t 