Full coverage: The South China Sea Issue

Tensions in the South China Sea have intensified due to maritime territorial disputes between China and the Philippines. China stands by its "indisputable sovereignty", saying it's based on historical evidence. Our reporter Han Bin spoke to Professor Wu Shicun, president of the National Institute for South China Sea Studies, who explained more about China's official position.

Collecting the historical evidence of Chinese sovereignty over the islands and reefs, in the South China Sea.

This has been the focus of Professor Wu Shicun’s work for two decades. The more he studies, the more he believes China has the best basis of territorial claims than other nations.

Manila's arbitration case is challenging China’s historic rights of the Dotted Line claims.

"The Dotted or Nine-Dash Line is closely related to sovereignty issues. Territorial issues are subject to general international law, not the UN Convention on the Law of the Sea," Wu said.

This institute has collected some rarely seen historical documents, which trace a period of history that is not well known.

Wu Shicun says there’s no official definition, but consensus has been reached on what the Dotted Line means.

"The Dotted, or Nine-Dash Line’, is a line of ownership of the features and historical waters. It indicates China’s claims of sovereignty over all the islands and reefs within the line, and its historical rights in fishing, navigation and exploitation," Wu said.

This Chinese map was drawn by the Ministry of Interior of the Republic of China in 1946. The 8 dashes mark what’s known as China’s "traditional maritime boundary line".

And this textbook published in 1936 marks the island groups within China’s domain, and China’s southernmost boundary at Zengmu Ansha, known as James Shoal in the West, at 4 degrees north latitude.

Wu Shicun stresses that China was the first country to discover, name and control these island groups. The history of continuous use and exercise of authority spans over 2,000 years.

This map identifies some island groups as "Wanli Changsha", literally meaning “long sandy banks tens of thousands of miles afar." They are marked as the territory of the Ming Dynasty, going back to the 17th Century.

Wu Shicun says no other country can provide more definitive evidence to support a claim. But today, the stakes are much higher.

"Due to various reasons, including the implementation of the UN Convention on the Law of the Sea, the pursuit for marine resources, and the increasing US pivot to Asia, the South China Sea disputes have developed from the original disputes over islands and waters, to a geopolitical contest of politics and interests, resource exploitation, and navigation control, involving both the claimant and non-claimant states, within this region and far beyond," he said.

Wu Shicun believes disputes over the South China Sea islands and their adjacent waters are unlikely to go away in the near future. He says putting aside disagreements and seeking joint development, would be a wise move.

The sovereignty over the island features lies at the heart of the South China Sea dispute and may become a flash point in the region. China believes to discuss the territorial claims; one must inevitably dig into their history. And these historical documents from China may help provide the context in which the current tensions are unfolding.