China has drilled deep in the South China Sea to retrieve sediment core from the seabed, amid tensions over disputed waters with rival claimants Taiwan and the Philippines, as well as with the US. Chinese scientists on a marine research vessel used China’s home-made “Sea Bull II” drilling system to obtain a sediment core 231 metres (253 yards)long at a depth of 2,060 metres (6,760 feet). The system can help explore natural gas hydrate resources in the seabed, referring to the solid icelike crystals formed from a mixture of methane and water that are touted as a promising source of energy.