The civilisation of Sindh and its cities remained hidden from the eyes of the world for some four millennia until its discovery at the beginning of the twentieth century. It reached its zenith between 2600 and 1900 BCE, after which it faded and disappeared for reasons unknown. Sindh's civilization belongs to a system of riverine cultures which grew out of an abundance of water and fertile soil. The latter gave rise to a rich mix of animal and plant life, which in turn promoted human settlement over a nomadic existence. This is one definition of civilization. During this period of civilization, more than one thousand Sindhi settlements covered over eight hundred thousand square kilometres, equivalent to modern-day Pakistan and northwest India. Despite their spread across vast swaths of land, the people of Sindh achieved a certain political unity which can be viewed in the similarity of their civilization, crafts, and material possessions. They constructed houses and public buildings out of mud and baked bricks, and had an unrivaled sanitation system, even in the time of the Roman Empire. The people of Sindh planted barley, wheat, and legumes and domesticated sheep, goats and water buffalo. They crafted stone seals, beads, and bracelets, and built boats which they used in long-distance trade. Yet the civilization of Sindh, which was a veritable social, cultural and political crucible, remains a "world shrouded in mystery" because Sindhi script has still not been deciphered to this day. This is despite the fact that almost one century has passed since the first archaeological discoveries and research into this world.