The migration of the Bantu is the first clearly defined movement of modern times. As we have shown, they began to move southward at least a thousand years before Christ, skirting the Congo forests and wandering along the Great Lakes and down to the Zambesi. What did they find in this land? We do not know certainly, but from what we do know we may reconstruct the situation in this way: the primitive culture of the Hottentots of Punt had been further developed by them and by other stronger Negro stocks until it reached a highly developed culture. Widespread agriculture, and mining of gold, silver, and precious stones started a trade that penetrated to Asia and North Africa. This may have been the source of the gold of the Ophir. The state that thus arose became in time strongly organized; it employed slave labor in crushing the hard quartz, sinking pits, and carrying underground galleries; it carried out a system of irrigation and built stone buildings and fortifications. There exists to-day many remains of these building operations in the Kalahari desert and in northern Rhodesia. Five hundred groups, covering over an area of one hundred and fifty thousand square miles, lie between the Limpopo and Zambesi rivers. Mining operations have been carried on in these plains for generations, and one estimate is that at least three hundred and seventy-five million dollars' worth of gold had been extracted. Some have thought that the older workings must date back to one or even three thousand years before the Christian era. "There are other mines," writes De Barros in the seventeenth century,[30] "in a district called Toroa, which is otherwise known as the kingdom of Butua, whose ruler is a prince, by name Burrow, a vassal of Benomotapa. This land is near the other which we said consisted of extensive plains, and those ruins are the oldest that are known in that region. They are all in a plain, in the middle of which stands a square fortress, all of dressed stones within and without, well wrought and of marvelous size, without any lime showing the joinings, the walls of which are over twenty-five hands thick, but the height is not so great compared to the thickness. And above the gateway of that edifice is an inscription which some Moorish [Arab] traders who were there could not read, nor say what writing it was. All these structures the people of this country call Symbaoe [Zymbabwe], which with them means a court, for every place where Benomotapa stays is so called."