The Indus Valley Civilization was one of the three earliest civilizations in the Old World, flourishing from 3300-1300 BC along the Indus River valley. At its peak, the civilization had over five million inhabitants living in well-planned urban centers with sophisticated sanitation systems. Though much remains unknown about the civilization, archaeologists have uncovered extensive artwork, advanced agricultural practices, a system of uniform weights and measures, and evidence of religious practices. The causes of the civilization's decline around 1800 BC remain unclear but may have included climate change or invasion.