The waters of Lake Chad sustain 70 million people in four countries. Beginning in the 1970s, the 25,000-square-kilometer lake began shrinking due to excessive drawdown for agriculture and mining. Now only 10 percent remains. The dwindling water supply devastated food production and fostered massive economic and political tensions. Many experts credit the worsening conditions for contributing to the rise of Boko Haram, an extremist group that has killed 20,000 people and forced 2.3 million more to flee.