SAN LUIS, Ecuador—Built near a spewing volcano, it was the biggest infrastructure project ever in this country, a concrete colossus bankrolled by Chinese cash and so important to Beijing that China’s leader, Xi Jinping , spoke at the 2016 inauguration. Today, thousands of cracks have emerged in the $2.7 billion Coca Codo Sinclair hydroelectric plant, government engineers said, raising concerns that Ecuador’s biggest source of power could break down. At the same time, the Coca River’s mountainous slopes are eroding, threatening to damage the dam.

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