A massive alluvial basin created by the Indus, Ganges, and their tributaries, fed by Himalayan meltwater. Flat, fertile, and continuously cultivable, it supports some of the highest population densities on Earth. Geography concentrates agriculture, cities, and state formation into a single east–west corridor. This plain is the demographic and political core of the subcontinent—easy to unify, hard to invade, and always capable of sustaining large, bureaucratic civilizations.
