The Harappan civilization flourished along the Indus River valley from around 2500 BCE to 1500 BCE. Major cities like Harappa and Mohenjo-Daro had populations over 35,000 people and sophisticated infrastructure like water and sewage systems. Though the exact beliefs are unknown due to a lack of burial sites, religion likely focused on fertility. The civilization engaged in regional trade and had uniform architecture, standards, and practices across its territory. However, by 1900 BCE the civilization was in decline due environmental degradation and deforestation, and cities were abandoned by 1500 BCE. The relationship between Harappan and later Indian civilizations remains unclear.