Crossing the Ob river to enter the Arctic Circle, travelling some 50km (31 miles) over ice. The way of life of the Nenets of the Siberian Arctic is inseparable from the reindeer. Every spring, they move enormous herds of reindeer from winter pastures on the Russian mainland, travelling more than 1,000km (620 miles) north to summer pastures in the Arctic Circle. This ritual is so old that it seems unclear whether the Nenets follow the reindeer, or vice versa. The migration starts in mid-March in freezing temperatures and is immediately challenged by the need to cross the frozen Ob river. But the Nenets take this in their stride, bolstered by a regimented work ethic and a robust culture. They survived early Russian colonisation of Siberia and the dark years of the Soviet regime, but are now being exposed to the perils posed by development of oil and gas fields in the far north.