The Indus Valley Civilization was a Bronze Age civilization that flourished between 3300 BCE and 1300 BCE in what is now Pakistan and northwest India. The civilization was spread out along the Indus River valley, and two major cities, Harappa and Mohenjo-Daro, had populations of up to 40,000 people. The civilization developed new techniques in metallurgy and handicraft and had a sophisticated urban culture with standardized brick sizes, street grids, and sewage systems. While the writing of the Indus civilization remains undeciphered, excavations have revealed evidence of advanced engineering and infrastructure. The decline of the Indus Valley Civilization around 1900 BCE may have been caused by environmental changes like