São Tomé would only become economically noteworthy with the introduction of a water-powered sugar mill in 1515, which soon led to the mass cultivation of sugar: "The fields are expanding and the sugar mills, too. At this time, only two sugar mills are here and another three are being built, counting the mill of the contractors, which is large. Similarly, the necessary conditions exist, such as streams and timber, to be able to build many more. And the sugar canes are the biggest I have ever seen in my life." Sugar plantations were organized with slave labor, and by the mid-16th century, the Portuguese settlers had turned the islands into Africa's foremost exporter of sugar.