Coal mine workers do the maintenance work at Qingshuiying Coal Mine in Lingwu City, northwest China's Ningxia Hui Autonomous Region, Feb. 18, 2018. A special team worked in the 800-meter-deep coal mine to guarantee the production safety during the Spring Festival holiday. (Xinhua/Wang Peng)

Coal mine worker Sun Lei talks with his workmates during lunch time at Qingshuiying Coal Mine in Lingwu City, northwest China's Ningxia Hui Autonomous Region, Feb. 18, 2018. A special team worked in the 800-meter-deep coal mine to guarantee the production safety during the Spring Festival holiday. (Xinhua/Wang Peng)

Coal mine workers do the maintenance work at Qingshuiying Coal Mine in Lingwu City, northwest China's Ningxia Hui Autonomous Region, Feb. 18, 2018. A special team worked in the 800-meter-deep coal mine to guarantee the production safety during the Spring Festival holiday. (Xinhua/Wang Peng)

Coal mine workers have lunch at Qingshuiying Coal Mine in Lingwu City, northwest China's Ningxia Hui Autonomous Region, Feb. 18, 2018. A special team worked in the 800-meter-deep coal mine to guarantee the production safety during the Spring Festival holiday. (Xinhua/Wang Peng)

Coal mine workers do the maintenance work at Qingshuiying Coal Mine in Lingwu City, northwest China's Ningxia Hui Autonomous Region, Feb. 18, 2018. A special team worked in the 800-meter-deep coal mine to guarantee the production safety during the Spring Festival holiday. (Xinhua/Wang Peng)