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Historians believe things started to fall apart around 1900BC. By 1700BC many Indus cities had been abandoned.

But how did this apparently peaceful, well-organised civilisation collapse in just 200 years?

Looking at the ruins we can see many changes. The cities became overcrowded, with houses built on top of houses. Important buildings like the Great Bath at Mohenjo-Daro were built over.

People stopped maintaining the drains and they became blocked. Some traders even hid their valuables under the floors of their homes. What were they scared of? Why did this happen?

• Trade was very important for the Indus civilisation. Their main trade partner was Mesopotamia, which was an advanced civilisation in the Middle East.

• Around the time the Indus cities started to fail, Mesopotamia was going through huge political problems.

• Mesopotamia's trade networks collapsed and this would have had a big impact on the Indus cities. There would have been less work for traders and for manufacturers, who made the things which the traders sold abroad.

• Some historians think this is why the cities collapsed.

• We know that only the cities fell into ruins. Farmers in the Indus Valley went on living in their villages and working on their farms, but the civilisation would never return to greatness again.

Some historians believed the Indus civilisation was destroyed in a large war. Hindu poems called the Rig Veda (from around 1500 BC) describe northern invaders conquering the Indus Valley cities.

In the 1940s, archaeologist Mortimer Wheeler discovered 39 human skeletons at Mohenjo-Daro. He believed that they were people killed by invaders.

Archaeologists now think this is not true. There is no evidence of war. Indus Valley people seem to have been peaceful. If they had an army, they have left few signs of weapons or battles.

It's more likely that the cities collapsed after natural disasters. Enemies might have moved in afterwards.

Many historians believe the Indus civilisation collapsed because of changes to the geography and climate of the area.

Movements in the Earth’s crust (the outside layer) might have caused the Indus river to flood and change its direction. The main cities were closely linked to the river, so changes in the river flow would have had a terrible effect on them. Repeated flooding may have led to a build-up of salt in the soil, making it hard to grow crops.

It is believed that at the same time, the Ghagger Hakra River (another river in the area) dried up. People were forced to abandon many of the cities located along its banks, such as Kalibangan and Banawali.

People would have starved and diseases would have spread. Perhaps because of this chaos, the rulers lost control of their cities. Lots of historians think it was a mixture of different reasons. Why do you think the Indus cities collapsed?

The Indus Valley lay forgotten and undiscovered for thousands of years.

In 1826, a British traveller in India called Charles Masson came across some mysterious brick mounds. He thought they looked like old castles, but didn't know who built them.

Thirty years later, in 1856, engineers building a railway found more bricks. They carted them off and continued to build the railway. These bricks were the first evidence of the lost Indus city of Harappa.

In the 1920s, archaeologists began to excavate the sites of Harappa and Mohenjo-Daro. They had uncovered the remains of two long-forgotten cities and found the Indus Valley civilisation.

The Indus Valley people did not leave great structures, like the Pyramids of Egypt or the Great Wall of China, but they did help shape the modern world.

Explore the pictures below to find out about the legacy of the Indus Valley culture.