The Indus Valley Civilization was located between the Indus and Saraswati rivers in modern-day Pakistan. It developed an advanced, urban society with large cities like Mohenjo Daro and Harappa housing up to 100,000 people each. The cities had sophisticated architecture, including multi-story brick buildings, and infrastructure like drainage systems and a grid layout of wide main streets and narrow alleys. The civilization engaged in extensive local and long-distance trade, though the reason for its eventual decline remains unclear, possibly due to flooding, landslides, or invasions.