The Oxus, that vast central Asian river that rises somewhere in the Afghan Pamirs, has fascinated explorers for centuries. Its name gives us the land of Oxiana. Yet few Europeans had set eyes on it before the second world war. Robert Byron’s 1937 book, The Road to Oxiana, is an account, among other things, of a failed attempt to find it.What most gripped the handful of 19th-century explorers, diplomats, spies and sportsmen who did make the perilous journey, however, was identifying its source. While the sources of other great rivers were being more or less accurately traced, that of the Oxus was fiercely contested, owing to the unusually difficult terrain. One of the worst journeys I have ever made was to one claimed source: Lake Victoria, or Syr Kul, on the Afghan-Tajik border (admittedly, I had been misdirected by the locals).