FULING, China — Tree-carpeted mountains rise high in this sleepy Yangtze River district, best known for its crunchy pickled mustard tubers. But one of these mountains is not like the others.

On the peak of Jinzi Mountain in Fuling, a single chimney stands sentinel over the adjacent Wu River. The chimney has been idle since it was built decades ago. Only in recent years has the public learned why.

Fifteen years ago, the local government announced that inside the hollowed-out mountain lay the remnants of what was once one of China’s most ambitious military infrastructure projects: the top-secret 816 nuclear plant.

Initiated in the 1960s during the height of tensions between China and the Soviet Union, the 816 project was China’s first attempt to build a nuclear reactor that could produce weapons-grade plutonium without Soviet involvement.