The rectangular structure was pulled by a 900-ton tug to a position just above the ill-fated ship. At 11:45 am, it was released and began to slowly sink.

Weighing 530 tons, the double-box structure is 35.7 meters long,14.4 meters wide and 12 meters high. It took engineers from Guangzhou Salvage Bureau more than a month to make.

The engineers have filled the gaps between the two steel boxes with sand, in order to increase the weight of the steel structure and cope with the effect of gravity under static pressure.

A score of frogmen assisted with underwater operations yesterday. The sunken ship is expected to be hoisted out of the seawater in July.

According to the salvage plan, the upper part of the steel structure will be brought out of the seawater together with the sunken ship, while the lower part will stay on the seabed forever, said Wu Jiancheng, who is in charge of the salvage operation.

Experts spent three years planning the salvage, considered to be a world first for underwater archaeology.

Archaeologists normally excavate the relics on the sunken boat first and then salvage the boat.

The sunken ship was found accidentally in 1987 by Guangzhou Salvage Bureau and a British underwater salvage company. The first ancient vessel to be discovered on the "Marine Silk Road" in the South China Sea, it was named "Nanhai No.1", meaning South China Sea No.1.

The ship is located some 30 nautical miles west of Hailing Island of Yangjiang City, and lies at a depth of more than 20 meters. About 30 meters long, the vessel is the largest Song Dynasty cargo ship ever discovered.

Archaeologists estimate that there are probably 60,000 to 80,000 relics on the sunken ship.

It is believed that a successful salvage of the sunken ship will offer important material evidence for restoring the "Silk Road on the Sea", and for the study of China's history in seafaring, ship-building and ceramics making.

According to Wu, workers have cleared away 25 tons of silt around the sunken ship and have brought out of the seawater 390 cultural relic items. They include green glazed porcelain plates, tin pots and shadowy blue porcelain objects.