'We were inspired by the Russians, who put a flag on the floor of the North Pole with their MIR [deep sea submarine],' said Zhao, an engineer at the China Ship Scientific Research Centre, who designed the hull of the submarine. 'It might provoke some countries, but we'll be all right. The South China Sea belongs to China. Let's see who dares to challenge that.'

China appears increasingly determined to exert its dominance over the resource-rich and strategically important waters, which are also claimed by a number of neighbours.

The submarine's name reflects the trend. When the project was launched in 2002 the submarine was called Ocean Base One. Later it changed to Harmony to echo the leadership's 'harmonious society' slogan. Now it is called Sea Dragon.

Zhao said the submarine had lived well up to its new name. With a maximum operational depth of 7,000 metres, it had surpassed the Russian MIRs and the deep sea probes of the United States, France and Japan.

The problem was that there was nowhere near China's coast that could provide a spot deep enough to test it.

'The closer to Philippines, the deeper the sea. We will put down national flags all the way until we reach their border,' Zhao said. 'And then we will go beyond and aim for the Mariana Trench.'

The Mariana Trench, the deepest spot on earth with a maximum depth of 11,034 metres, lies to the northeast of the Philippines.

Zhao said the submarine's main task was to conduct mineral surveys.

'We have seen lots of strange marine creatures at the bottom of the South China Sea, such as shrimp, crabs and fish, which all seemed blind but moved surprisingly fast when touched, and yet they were not what we came all the way for,' he said. 'We were looking for minerals.'

Armed with two delicate robotic arms developed by the Chinese Academy of Sciences' Shenyang Institute of Automation that mimic the function of human arms, the Sea Dragon usually dived down to the sea bed with a basket in one hand and a shovel in the other. The crew had been trained to detect signs of exposed mineral veins on sea ridges and take samples.

The Sea Dragon also visited hydrothermal vents produced by active volcanoes because rare minerals were often concentrated there.

'We were like gold prospectors venturing into an area never explored before,' Zhao said. 'We were hit by surprises every day.'

Every hour underwater, however, costs a fortune. Increasing with depth, the submarine's hourly operational cost starts from 100,000 yuan (HK$114,000). The submarine itself was expensive, too. So expensive, in fact, that the central government was shocked by the budget and planned to postpone the project. But with tension over the South China Sea increasing, the government has now decided to build another Sea Dragon.

'A lone dragon has an irremediable shortcoming. If something malfunctions and it remains at the bottom, the crew will die,' Zhao said. 'A double-dragon would be invincible.'

Hou Deyong, another designer at the centre, said that the Sea Dragon was a civilian submarine built for scientific purposes. 'We are unarmed and mean no threat,' Hou said.

The Sea Dragon needs the support of the People's Liberation Army Navy, according to Zhao. 'The navy has escorted all our previous missions and I think they will continue to do so,' he said. 'The further we go, the more we need guns to protect ourselves.'

Deep significance

A scientific submarine has explored unknown terrain at a depth of more than 3,700 metres

Number of civilian crew members was: 3