When the early farmers moved down from the hills into the valleys of the Nile, Tigris, and Euphrates Rivers, they had to develop the technology of irrigation. The freshwater rivers passed through lands essentially barren due to the lack of rainfall. Through irrigation, water could be removed from the rivers and spread on the nearby land, making it bloom. Ever since these early days, irrigation has been an important tool of farmers, making other deserts bloom as well.
Modern examples are the very productive deserts of California.
Irrigation increases the food value of River, Grassland, Plains, Desert, and Hill squares. When thinking humans first evolved, it was possible to find nearly pure deposits of several metals exposed on the Earths surface. The most common were gold, silver, and copper. As time passed, the surface deposits were exhausted and men began digging into the ground looking for more. The mining and excavation of the Earth in search of minerals continues today, although the list of things for which we dig has greatly expanded.