The Indus Valley civilization flourished from 3300 BCE to 1300 BCE along the Indus River valley. It was a large, well-organized urban society that had widespread trade networks. The civilization included over 1,000 cities and settlements, some as large as 40,000 people. It had advanced sanitation systems, including covered sewers. Writing systems and uniform weights and measures showed a sophisticated culture. The civilization unexpectedly declined around 1300 BCE possibly due to climate change and flooding, though the exact cause is unknown.