Tourists visit an ice cave in Luyashan scenic area in Ningwu County of Xinzhou City, north China's Shanxi Province, May 4, 2019. According to research, the ice cave with a depth of 100 meters came into being 3 million years ago. (Xinhua/Cao Jianguo)

Tourists visit an ice cave in Luyashan scenic area in Ningwu County of Xinzhou City, north China's Shanxi Province, May 4, 2019. According to research, the ice cave with a depth of 100 meters came into being 3 million years ago. (Xinhua/Cao Jianguo)

Tourists visit an ice cave in Luyashan scenic area in Ningwu County of Xinzhou City, north China's Shanxi Province, May 4, 2019. According to research, the ice cave with a depth of 100 meters came into being 3 million years ago. (Xinhua/Cao Jianguo)

Tourists visit an ice cave in Luyashan scenic area in Ningwu County of Xinzhou City, north China's Shanxi Province, May 4, 2019. According to research, the ice cave with a depth of 100 meters came into being 3 million years ago. (Xinhua/Cao Jianguo)

Tourists visit an ice cave in Luyashan scenic area in Ningwu County of Xinzhou City, north China's Shanxi Province, May 4, 2019. According to research, the ice cave with a depth of 100 meters came into being 3 million years ago. (Xinhua/Cao Jianguo)

Tourists visit an ice cave in Luyashan scenic area in Ningwu County of Xinzhou City, north China's Shanxi Province, May 4, 2019. According to research, the ice cave with a depth of 100 meters came into being 3 million years ago. (Xinhua/Cao Jianguo)