"As a child I always imagined that the earth had a heart and supposed that it must lie in the core of Asia." Colin Thubron, the celebrated writer, follows the traveller's old obsession - to find the centre of the earth. He takes the Silk Road from the ancient Chinese capital at Xian along the northern rim of the fearsome Taklamakan desert and into the remote mountains along the border between China and Russia. The Turkomen traders and farmers, whose ancestors made the Silk Road work, are still there, but they are not native Chinese. Holding fast to Islam, these forgotten people are virtually a nation on their own.