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The cities of the ancient Indus were noted for their urban planning, baked brick houses, elaborate drainage systems, water supply systems, clusters of large non-residential buildings, and techniques of handicraft and metallurgy. Mohenjo-daro and Harappa very likely grew to contain between 30,000 and 60,000 individuals, and the civilisation may have contained between one and five million individuals during its florescence. A gradual drying of the region during the 3rd millennium BCE may have been the initial stimulus for its urbanisation. Eventually it also reduced the water supply enough to cause the civilisation's demise and to disperse its population to the east.