The Indus River Valley civilization lasted for over 1000 years until disappearing mysteriously around 1750 BC. Two major cities, Mohenjo-Daro and Harappa, had populations of 35,000 to 40,000 people and featured sophisticated planned cities with grid layouts, fortified citadels, advanced plumbing and sewage systems in most homes, and public wells. The stable society engaged in trade and showed few signs of social divisions or conflict.