Around 2600 BCE, the cities of the Indus Valley were among the most advanced on Earth. Mohenjo-daro and Harappa had planned streets, standard-sized bricks, covered drains and public wells. Weights and seals were consistent across hundreds of kilometres. Then, from about 1900 BCE, the big cities emptied. Writing disappeared. Long-distance trade dried up.
The Great Bath at Mohenjo-daro, Sindh, Pakistan. Photo: Saqib Qayyum via Wikimedia Commons (CC BY-SA 3.0), cropped.
There was no single dramatic end, and that's partly why the question is still argued over. Here are the main explanations historians and archaeologists have put forward, and how well each one holds up.
1. The invasion theory
In the mid-20th century, archaeologist Mortimer Wheeler argued that invading Indo-Aryans destroyed the cities. He pointed to skeletons found lying in the streets at Mohenjo-daro as victims of a massacre.
This idea has been largely abandoned. Later study showed those skeletons came from different time periods, not a single event, and there's no layer of widespread burning or battle damage you'd expect from a conquest. The cities declined slowly over generations.
2. Climate change and a weakening monsoon
This is now the leading explanation. Around 4,200 years ago, a period of drier conditions affected regions from Egypt to Mesopotamia to South Asia, often called the 4.2 kiloyear event. Evidence from lake sediments and cave formations suggests the summer monsoon weakened over the following centuries.
For a farming civilisation, less reliable rain meant smaller harvests and less surplus to feed large cities. The pattern fits what archaeologists see: people didn't vanish, they moved east and south toward wetter areas and lived in smaller villages.
3. Rivers that changed course or dried up
Many Indus sites sit along the dried bed of the Ghaggar-Hakra river system, which some link to the Sarasvati of the Vedas. As this river system lost water, settlements along it would have struggled. Other researchers point to shifting channels of the Indus itself and to tectonic activity in the region, which could reroute rivers and leave towns stranded.
4. Floods
At Mohenjo-daro, layers of silt suggest repeated flooding. Some scholars think heavy floods, possibly linked to river changes, damaged the city and made rebuilding harder each time. This probably explains local trouble rather than the decline of the whole civilisation.
5. The collapse of trade
Indus seals and goods have been found in Mesopotamia, and Mesopotamian texts mention a trading partner called Meluhha, widely identified with the Indus region. Around the same time the cities declined, that trade fell away. Losing a major market would have hit the merchants and craftspeople who made the cities rich, though it was likely a symptom of wider stress as much as a cause.
6. Disease and stress
Studies of skeletons from late Harappa show more signs of injury and infectious disease than earlier burials. Crowding, failing harvests and poorer water conditions could all have worsened health and made city life less attractive.
So what really happened?
Most specialists now think there was no single cause. A drying climate put pressure on farming. Rivers shifted. Trade networks weakened. Cities that depended on surplus food and long-distance exchange became harder to sustain, and people adapted by spreading out into smaller rural communities.
That's less a collapse than a transformation. Indus traditions in farming, crafts and possibly beliefs carried on in the villages that followed. The great cities ended. The people didn't.
We still can't read the Indus script, and until someone does, some of the civilisation's own explanation for its story stays out of reach.
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