What would be a likely natural feature in an extremely densely populated area of the world

A likely natural feature in an extremely densely populated area would be a well watered, fertile plain. An example is the Northern plain of India also called the Indo - Gangetic plain. It s on one of the world's most densely populated area per square kilometer. This plain is formed from the flood plains of three great river systems - the Indus, the Ganga and the Brahmaputra. The soil cover in the plain is rich and fertile and water is found in plenty. These factors combined with a favorable climate makes it one of the most intensely farmed and populated areas in the globe.

Its decreasing because of illnesses