It all comes down to black gold. Anti-Japan protests erupted in at least 100 Chinese cities on Tuesday, as anger over a struggle for control of oil and gas in the East China Sea turned violent, and increased the tension between the countries.

The focal point is a dispute over a remote island chain known as Senkaku in Japan and Diaoyu in China, lying east of China and south-west of Japan (see map). The US handed them to Japan after the second world war, but China says that it has a prior claim. Japan recently purchased several of the islands back from a private Japanese owner, and their nationalisation has ratcheted up anger in China.

“There is potential oil and gas,” says Pui-Kwan Tse of the US Geological Survey. It’s not clear how much is there, or whether it would be economical to drill for it.

However, the East China Sea is rich in oil and gas reserves, many of which have only been discovered in recent decades. China and Japan are both eager to stake a claim: China’s energy demand is growing rapidly, and Japan’s reserves are limited.