Photo taken on Aug 2 shows the beautiful landscape of the underground forest in Mudanjiang, northeast China's Heilongjiang province. A lot of tourists come here to enjoy the fresh air and cool sensation in hot summer. (People's Daily Online/Huang Jin)

The underground forest is also known as the "crater forest". According to scientists, the volcanic eruptions about 10,000 years ago form the rare sunken terrain of the "underground forest".