Sustainability: In the East Kolkata Wetlands, the city’s waste is purified and turned into fish food. But now rapacious property developers are threatening its future. Amitangshu Acharya and Sudipto Sanyal investigate this worrying story for The Economist's 1843 magazine. "The most extraordinary thing about the wetlands, however, is their capacity to transform the sewage of a teeming metropolis into shimmering black gold. The wetlands have been variously described as the 'kidneys of Kolkata', 'the lungs of the city' and an 'indispensable heritage'. Every day 750m litres of sewage and wastewater flow into them. With nearly a century of knowledge behind them, each year some 50,000 fisherfolk and vegetable farmers turn excrement into 10,000 tonnes of fish and 50,000 tonnes of vegetables."