For centuries, Timbuktu was Europe's idea of the unreachable place - an African El Dorado where the roofs were said to be made of gold. The myth had roots in something real: in 1325 the Malian emperor Mansa Musa travelled to Mecca with so much gold that his spending in Cairo devalued the Egyptian currency for years afterwards. By the time European explorers went looking, the city had been in decline for two centuries. Nobody in Europe knew that, because nobody had been there and come back.

In the 1820s, the French put up a prize: a small fortune for the first person to reach Timbuktu and return alive to describe it.

Two men went after it, and they could not have been less alike. Alexander Gordon Laing was a Scottish army officer who crossed the Sahara openly. René Caillié was the son of a convict, orphaned at eleven, self-taught and s ...