Photo taken on May 13, 2019 shows the inside view of Wanxiang karst cave in Hanwang Township of Longnan City, northwest China's Gansu Province. (Xinhua/Tu Guoxi)

Photo taken on May 13, 2019 shows the inside view of Wanxiang karst cave in Hanwang Township of Longnan City, northwest China's Gansu Province. (Xinhua/Ma Ning)

Tourists view Wanxiang karst cave in Hanwang Township of Longnan City, northwest China's Gansu Province, May 13, 2019. (Xinhua/Ma Ning)

A tourist takes pictures in Wanxiang karst cave in Hanwang Township of Longnan City, northwest China's Gansu Province, May 13, 2019. (Xinhua/Ma Ning)

A tourist takes pictures in Wanxiang karst cave in Hanwang Township of Longnan City, northwest China's Gansu Province, May 13, 2019. (Xinhua/Tu Guoxi)

Tourists view Wanxiang karst cave in Hanwang Township of Longnan City, northwest China's Gansu Province, May 13, 2019. (Xinhua/Ma Ning)