In the plains of the Indus, which flooded twice a year, providing fertile grounds for abundent agriculture, the largest of the ancient civilizations resided, the IVC (3300 BCE - 1300 BCE).

They had great cities, like Mohenjo-daro and Harappa, perpendicular street, multi-story building, and written language (which we don't know how to read). In short, they were a developed society.

They even had a sewage and plumbing system.

Their biggest building was what is now known as the Great Bath.

They had a caste system, splitting the population among groups such as priests, warriors, merchants, or peasants.

They practised trade, using seals as identification numbers on products. Their products even reached Mesopotamia, while archeologists also found bronze in the Indus Valley, which is not native to the region (so it must have been brought or traded).

They were also, according to archeological findings, a peaceful society.

Their population started declining into obscurity around 1750 BCE. There are three theories as to why:

1. Conquest: their peaceful society may have been overrun by the people from the Caucasus.
2. Environmental disasters: they may have destroyed the environment themselves through their practises.
3. Earthquakes: they may have changed the course of the river enough to dry out the surrounding areas.

The exact reason is unclear.