Photo taken on June 2, 2011 shows an overhead view of the underground operation floor of the Three Gorges Dam hydropower plant in Yichang, central China's Hubei Province. A second underground turbo-generator, namely No.31 Power Unit, was put in operation on Thursday, following the launch of No.34 Power Unit on May 24, 2011. The Three Gorges' underground power station is designed to consist of six 700,000-kilowatt turbo-generators which are expected to generate a total of 3.5 billion kilowatt hours of electricity annually when all of them are put in use in two years' time. [Photo: Xinhua]