Why were these Indus Valley ruins left abandoned in India?

Thousands of years ago, Ancient India prospered economically along the Indus valley region as a decentralised society. Then, one day, these decorated cities were left abandoned and forgotten for thousands of years. During the 19th century, British explorers wandered the Indus valley plains to rediscover numerous ruins that once promoted a self efficient, high status civilisation.

Unfortunately, there is no written evidence prior to 2nd millennium BC, the invention of grid design town planning. No archaeologist has come up with the right answers as to why these urban cities were built near the Indus river.

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Acknowledgements:

Images from Google earth used for educational purposes only

Introduction to the Indus valley (harappan) civilisation video by History with Cy
https://www.youtube.com/watch?v=pKGq56YP98I

Domain 2 early Asian civilisations lesson 1 Indus river valley part 1 video by Mario Woytko
https://www.youtube.com/watch?v=SUzeSz9hSTw

Brief history of Indus valley (harappan) civilisation video by 5 minutes
https://www.youtube.com/watch?v=r-vbm9Hifw0

Music by PenguinMusic,Serge Quadrado, ShidenBeatsMusic, AntipodenWriter, Imgmidi and Alex MakeMusic from Pixabay

Videos by Sachin Ghatmale and Guru Amirthalingam from Pixabay