Indus Valley Declines
Around 1900 BCE, the great cities of the Indus Valley civilization, also called the Harappan civilization, began a long decline that transformed one of the ancient world's largest urban societies into a scattering of smaller rural communities. The change matters because the Indus people built remarkably planned cities without leaving behind any deciphered writing, so their fate remains one of archaeology's enduring puzzles. The date is approximate, and the process unfolded over several centuries rather than in a single moment.
At its height, from roughly 2600 to 1900 BCE, the civilization spread across what is now Pakistan and northwestern India, covering an area larger than Egypt or Mesopotamia. Cities such as Mohenjo-daro, Harappa, Dholavira, and Rakhigarhi featured grid-like streets, standardized baked bricks, covered drains, and sophisticated water management. Standardized weights and carved stamp seals point to organized trade, which reached Mesopotamia by land and sea. The undeciphered script, found mostly on seals, has resisted every attempt at translation.
Evidence for the decline comes from archaeological surveys and excavations. Major cities shrank or were gradually abandoned, long-distance trade faded, and the use of seals and the script disappeared. Population seems to have shifted toward the east and south, into the Ganges plain, Gujarat, and the foothills of the Himalayas, where smaller villages relied on a mix of crops and local exchange. Older claims that invaders destroyed the cities have largely been abandoned, since excavations show no clear evidence of a violent conquest.
Most current explanations point to environmental change. Researchers have proposed that the monsoon weakened over this period, making agriculture in the floodplains less dependable. Others suggest that rivers shifted course. The Ghaggar-Hakra system, which some link to the legendary Sarasvati, may have lost much of its water, though scholars disagree about how large that river was and when it dried. Floods at some sites, disease, and breakdowns in trade networks have also been suggested. Many historians favor a combination of causes, with the effects varying from region to region.
The legacy of the Indus civilization is still being uncovered. Its cities were forgotten until the 1920s, when excavations at Harappa and Mohenjo-daro revealed a lost culture. Some features of later South Asian life, such as certain craft techniques, motifs, and aspects of urban planning, may have roots there, though the links are debated. Until the script is deciphered, the story of how and why the Harappan world faded will rely on bricks, pottery, bones, and sediments rather than the voices of its people.