Black slaves were imported into the Muslim world from Africa by a number of routes northward across the Sahara desert, and by sea into Arabia and the Persian Gulf. Estimates of the number involved vary greatly but it seems that there may easily have been 10 million, perhaps even twice that number. Two-thirds of African slaves were female. The males were considered to be troublesome. An uprising of slaves from West Africa, the Zanj, who had been imported into the Tigris-Euphrates delta to reclaim salt marshland through their backbreaking labour, lasted from 869 until 883. The mortality rate was very high because of the harsh conditions, but the trade was so lucrative that merchants were not deterred by the numbers who died. Harrowing eye witness accounts tell of the vast scale and miserable conditions of the slave trade in Africa. In the 1570s many thousands of black Africans were seen for sale in Cairo on market days. In 1796 a caravan was seen by a British traveller leaving Darfur wit