Pakistan: The Indus Valley Civilization, one of the earliest urban cultures, with cities such as Harappa and Mohenjodaro, flourished for 600 years during a climatic "Goldilocks" period—not too wet, not too dry. According to a new study of river and flood deposits, when the civilization first began to grow, around 4,500 years ago, the monsoon rains were declining and flood intensity decreased enough to make intensive agriculture possible. The monsoon continued to weaken over time, and the increasing aridity might have contributed to the abandonment of the great Indus cities around 3,900 years ago.