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They are often sited as being one of a relatively small group of civilizations that kick started itself from small groups of hunter gatherers into a large well organized agricultural system. They were capable of advanced urban and agricultural planning. Earlier periods of drought were the impetus for sophisticated systems of irrigation. They were involved in extensive trade with their Mesopotamian (Sumerian) neighbors.
Rediscovered starting in the 19th century, it joins a long lines of ones very powerful civilizations which were almost forgotten after their collapse. The name Harappan is derived from the village of Harappa in India, where some of the first discoveries were made.
The exact mechanism of their collapse is a little unclear. In a review of a general text on their culture it is discussed:
Rise and fall of the Indus Civilization
Earlier theorists had suggested that they were wiped out by invasion. A theory that is generally not popular with archaeologists.The Indus civilization began with some major developments like the introduction of writing and a surprisingly uniform culture over the whole of the greater Indus valley. According to Parpola this development was due to increased maritime trade and closer cultural contacts with Mesopotamia and the Gulf region. There is now general agreement that Meluhha mentioned in the cuneiform inscriptions refers to the land of the Indus. Parpola lays stress on the importance of Harappan contacts with West Asia, which provide relevant parallels and potential sources of information on the Harappan culture.
The Indus Civilization flourished between about 2600 and 1800 BC when it collapsed into regional cultures at the Late Harappan stage. According to Parpola the collapse was due to a combination of several factors like over-exploitation of the environment, drastic changes in the river-courses, series of floods, water-logging and increased salinity of the irrigated lands. Finally the weakened cities would have become easy victims of the raiders from Central Asia, whose arrival heralded a major cultural discontinuity in South Asia.
On the basis of extensive explorations carried out in Northern Mesopotamia, a joint French-American team led by H. Weiss of Yale University has determined that most of the old world civilization were severely affected by a prolonged drought that began about 2200 B.C. and persisted for about 300 years. The most drastically hit region seems to have been the Akkadian civilization neighbouring India. The drought may have been triggered by massive volcanic eruptions. According to the findings of this historic study concluded only recently
"At approximately 2,200 B.C., occupations of Tell Leilan and Tell Brak (in Northern Mesopotamia) were suddenly abandoned...a marked increase in aridity and wind circulation, subsequent to a volcanic eruption, induced considerable degradation in land use conditions.... this abrupt climatic change caused abandonement of Tell Leilan, regional desertion, and collapse of the Akkadian empire based in southern Mesopotamia. Synchronous collapse in adjacent regions suggests the impact of abrupt climatic change was excessive."
An end uncannily like that of the Harappans. The authors of this momentous study note that the collapse of the Akkdians more or less coincided with similar climate change, land degradation and collapse noted in the Aegean, Palestine, Egypt, and India. The date of 1900 BCE given by S.R. Rao for the collapse of the Harappans should be seen as approximate. More accurate methods are now available that show this date to have been sometime before 2000 BCE, and they are well within the calibration error of radiocation and other scientific dating techniques.
The basic point is: as a result of several independent explorations conducted over a vast belt from southern Europe to India, it is now clear that civilizations over a large part of the ancient world were brought to a calamitous end by an abrupt climate change on a global scale. To attribute a global calamity of such colossal magnitude to nomadic 'Aryan' tribes is simplistic in the extreme.
The climactic connection may have been indirect.
Another recent study that looked very closely at the pollen records, etcetera, noted the following:
Marco Madella, Dorian Q. Fuller, Quaternary Science Reviews 25 (2006) 1283–1301
No climatic event can be blamed for a precipitous end of this civilisation, although strategic local shifts in agriculture that may have begun in response to prolonged droughts at ca 2200BC may have contributed to the de-urbanisation process and the restructuring of human communities over the following 200–300 yr.
I am not sure why this would be such a mystery. Most of the long lasting ancient civilizations were located within a relatively stable area of agricultural production. Situation along a river valley was common, as it made it possible to survive extended drought periods by use of sophisticated irrigation techniques.
But the surrounding areas, where other groups had grown and organized, did not have this stability. So when a drought, or cold spell hit their area, they starved. When they starved, they looked around at who still had food, and marched on them. Egypt was attacked by Sea People, and the people of the Indus River Valley were likely attacked (probably not by Aryans) themselves. In the case of the Egyptians their agricultural hinterland was somewhat remote to the attacking Sea Peoples. The Indus Valley was likely not so lucky. An analogous situation can be seen with the collapse of the Western Roman Empire. It was finally done in permanently when the Vandals were able to reach their North African grain producing areas. The Eastern Roman that required rediscovery. Empire's hinterland was not reached.
Regardless, you have yet another major civilization requiring rediscovery.