Economic Factors According to another theory, if trade had triggered
growth and prosperity of the Harappans, it also sounded the death-knell for
this civilisation. A steep-decline in trade causing a severe economic
depression may have turned many urban centers into ghost towns. Besides,
archaeologists have reported evidence of street encroachments, a breakdown
in sanitation and massive squatter colonies, indicating socio-cultural erosion.
Survival and Continuity The Harappans who survived the natural
calamities and the socio-economic erosion may have moved southwards,
finally adopting the culture of their new homelands. The tales of destruction
by floods and natural calamities may have passed into folklore and then into
the myths of succeeding generations. The loss for the Harappans was the gain
for the Gangetic plains, where the stage was set for an equally memorable
period in ancient Indian history. Part of the foundation for that was laid by
the survivors of the Harappan civilisation.
            ARYAN INVASION/MIGRATION THEORY
  The traditional view was that the Harappans were destroyed by invading
  Aryans. But there is no evidence of any such violent end. Then came the
  theory that immigrants from the north entered the regions, forcing the
  others to flee southwards.
  The idea that Mohenjodaro fell to a group of invaders is based on two sets
  of archaeological data. In the upper levels of the eastern mound at
  Mohenjodaro, there are about thirty-three randomly distributed skeletons
  of males, females and children.
  These skeletons are not contemporary; in one case, it has even been
  established that the cut mark which the skeleton bears did not lead to
  death. This massacre idea is a myth.
  In different areas of the north-west, there are a few archaeological objects
  which are of west and central Asiatic derivation. They were not found in
  well-defined archaeological contexts. There is no reason why they should
  be called contemporary and indicate incoming invaders.
  However, such objects were forcibly interpreted that way and along with
  the above-mentioned skeletons, led to the hypothesis that the Indus
  civilisation in general and Mohenjodaro       in particular, fell prey to a group