The Indus Valley Civilization flourished along the Indus River in modern-day Pakistan from around 1900 BC to 1700 BC. They built large, well-planned cities laid out on a grid pattern with advanced sewage systems and indoor plumbing not seen elsewhere. The civilization cultivated cotton and was the first to weave cotton cloth. While highly advanced, the civilization suddenly declined around 1700 BC for unknown reasons, possibly due to invasion by outsiders, changing weather patterns, or a combination of factors that weakened the society.