Final answer:
The Indus Valley civilization's decline may have been caused by environmental degradation, climate change, tectonic activity, disease, and vulnerability to invasion.
Step-by-step explanation:
Current theories about the disappearance of the Indus Valley civilization comprise a range of possibilities. Environmental degradation, likely due to intense agricultural practices leading to salinization and deforestation, is one significant factor that contributed to its decline. Additional theories include climate change affecting the monsoon patterns and water sources, tectonic activity altering river flows, and epidemic disease. Moreover, the civilization may have been weakened by these environmental challenges, making it easier for invading groups like the Aryans to have an impact. The decline seems to have taken place slowly over several centuries, finally leading to the abandonment of cities and the collapse of their sophisticated urban culture by around 1500 BCE.