Between around 3500 BC and 2000 BC, people in the Indus Valley built more than 100 towns. The largest were Mohenjo-Daro and Harappa, with populations of 40,000. These towns had large temples, granaries, brick houses, and streets laid out in neat grid patterns.
Farmers grew wheat, barley, cotton, and rice on land fertilized by yearly Indus River floods. They also raised animals. In towns, people made cloth, pottery, metalwork, and jewelry. On the coast, they went abroad to trade.
At Mohenjo-Daro, the Indus River changed its course, causing a water shortage. Other towns may have been destroyed by floods, disease, or invaders. But nobody knows for sure why the Indus Valley civilization collapsed.