The Indus Valley civilization flourished around 4000-1000 BC along the Indus River in modern-day Pakistan. Two major cities, Harappa and Mohenjo-Daro, had sophisticated urban planning with standardized bricks, streets, wells, and drainage systems. The civilization engaged in extensive trade networks. While its writing remains undeciphered, artifacts indicate it may have been an advanced civilization. Around 1500 BC, ecological disasters and Aryan migrations contributed to the decline and deurbanization of the Indus Valley civilization.