The Indus Valley Civilization was a vast and highly advanced ancient society known for its planned cities, sanitation systems, and trade networks. Centered around cities like Harappa and Mohenjo-Daro, it thrived around 2600–1900 BCE. Despite its sophistication, its undeciphered script leaves its culture largely unknown. Its decline was gradual, likely caused by environmental changes such as shifting rivers and climate shifts, making it one of history’s most mysterious civilizations.