A bird's eye view of one sinkhole surrounded by dense forest in Hanzhong, Shaanxi province. [Photo provided by chinadaily.com.cn]

Experts have confirmed that a group of sinkholes discovered Hanzhong, Shaanxi province in Northwest China, contains the largest number of the rare geological phenomenon in the world.

The Shaanxi provincial government announced on Thursday that an investigation revealed there were 49 sinkholes in the area, the most in the world.

According to head of Shaanxi's land resources department, Wang Weihua, it took four months to inspect the 5,000-square-kilometer area of the sinkhole group, with 49 sinkholes found in the core area of more than 600 square kilometers.

The area comprises one super large sinkhole, as well as 17 large and 31 conventional sinkholes.

The sinkhole group was found at the Qinling-Bashan Mountains spread over the counties of Ningqiang, Nanzheng, Xixiang and Zhenba in Hanzhong city.

The highest number were found in Zhenba at 19, while 16 are within Nanzheng's borders, Wang said.

"The largest one was in Zhenba, which had the largest diameter of 520 meters and a maximum depth of 320 meters," Wang said.

Geologists say that sinkholes are formed when carbonate rocks are eroded for a long time by an underground river which hollow out the rocks which eventually collapse.

It is a rare geological phenomenon that can take up to 500,000 years to form.

Before the discovery of the Hanzhong sinkhole group, only about 130 sinkholes had been found and confirmed in the world, geologists said.

At the end of October, Shaanxi provincial government organized a group of foreign and Chinese geologists to conduct a field investigation. The consensus of the geologists was that the Hanzhong sinkholes were not only the largest in the world in terms of number, but also excellent in typicality, rarity and integrity. The sinkholes are expected to be of great scientific value and could become a tourist drawcard.

Renowned French cave explorer Jean Poutasi inspected the sinkhole in Ningqiang county and praised it as "the world's most beautiful sinkhole".

Zhang Yuanhai, a senior engineer with Institute of Karst Geology under Chinese Academy of Geological Science, said that the scientific value of Hanzhong sinkholes was unmatched.

"The sinkholes found before are located between north latitude 24 degrees and 31 degrees, but Hanzhong sinkholes are located near north latitude 33 degrees, and the first geological landscape found in the most north boundary of the humid tropical and subtropical Karst landform area, which has exceeded expectations of all geologists," Zhang said.

Zhang considered Hanzhong sinkholes as a world wonder which would fill gaps in the research of karst geology, and also provide more original samples for biological research.

Liu Tongliang, director of the Institute of Karst Geology at the Chinese Academy of Geological Science, said that the large-scale sinkhole group provided valuable material for the research on the formation of the Hanzhong basin and cave canyon. It would also help scientists to understand the evolution of the ancient environment in the south of Qinling Mountains, as well as the ancient climate change of North and South China and the world.

The provincial government said that Shaanxi would develop a comprehensive plan to better protect the "Earth's gift and the world's wonder" for all mankind.