Underwater robots, unmanned ships and sonar detectors have been deployed, in Zhuhai in a bid to rescue the group, the city’s vice mayor Zhang Yisheng said.
He told a news conference that the water level in the tunnel had dropped by 11.3 metres (37ft), according to an online report by state broadcaster CCTV.
Zhuhai is a coastal city in Guangdong province and near Macao at the mouth of the Pearl River delta. It was one of China’s early special economic zones when the ruling Communist Party started opening up the nation’s economy about 40 years ago.
More than 1,000 workers, 22 fire trucks and five pumping vehicles were deployed as part of the search and rescue mission three days ago, with search and rescue teams dispatched from surrounding cities in Guangdong province.
Search teams have been slowly advancing into the tunnel as water is pumped out. As of Sunday morning, they were about 600 metres into the tunnel, a little more than half the 1.1-kilometre distance to where the workers were trapped.
Their advance has been slowed by carbon monoxide fumes from machinery being used in the tunnel as part of the operation, though the level of the potentially deadly gas has been lowered by improving ventilation.
The flood happened at about 3.30 am on Thursday. An abnormal noise was heard and bits of material started falling off on one side of the two-tube tunnel.
An evacuation was ordered but water rushed in and flowed through a connection into the other tube of the tunnel, trapping 14 workers on that side.
The tunnel lies under a reservoir, but the cause of the accident is still under investigation.
Additional reporting by The Associated Press
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