The water level in the Dhauliganga river surged again briefly on Thursday, forcing rescue workers to stop for some time their operation to reach the over 30 people trapped in the Tapovan hydel project tunnel after the flash flood four days ago.
The interruption took place amid a massive effort by multi-agency rescuers to break through the sludge and debris that choked the tunnel after Sunday's disaster, which left 35 dead and about 170 missing.
Later the operation resumed but officials said they were only sending smaller teams for now to the rescue site.
Hours before this sudden rise in the water, rescuers had also begun an operation to drill through the debris from the mouth of the tunnel to reach life-saving devices to the trapped workers, an apparent change in strategy after trying to shift mounds of debris.
But as information on the rise in water level upstream came, rescue workers scrambled out of the tunnel with their heavy machinery. A joint press briefing by the rescue agencies also came to sudden halt.
Chamoli District Magistrate Swati S Bhadauria said work was temporarily halted as a precautionary measure.
Amid mounting concern over the lives of those inside, the focal point of the rescue operation remains the 1.5-km "head-race tunnel" -- a part of the 2.5-km long network of tunnels.
"A drilling operation was started by the rescue teams at 2 am to peep into the slush-flushing tunnel that is about 12-13 metres below," Vivek Kumar Pandey, the spokesperson for the lead rescue agency, Indo-Tibetan Border Police, said in Delhi.
As the continuous flow of slush and silt remains a major obstacle between the rescuers and those trapped inside, a boring operation by a huge machine is being undertaken to see if this problem can be addressed in a different way and the teams can go further deep inside, he added.
Drilling through the debris started from around 68 metres inside the tunnel, Garhwal Commissioner Ravinath Raman, who is supervising the massive rescue effort at the National Thermal Power Corporation project site, told PTI.
The focus at the moment is to take life-saving devices such as oxygen cylinders to those trapped by drilling, Raman said.
The drilling is to be done for 12 metres to access the probable location of those trapped, he added.
Till Wednesday, about 120 metres of slush from the mouth of the tunnel was cleared and those trapped inside were stated to be located somewhere at 180 metres, where the tunnel takes a slip or a turn.
"More slush and water coming from inside the tunnel is making the way ahead difficult," Pandey had said on Wednesday.
ITBP chief S S Deswal had told PTI on Wednesday that his men and those from other agencies will continue with the rescue operation till "any length of time" or till a logical conclusion is arrived at and the trapped workers are located.
He had expressed hope that those trapped inside might be safe with the help of possible air vents in the structure.
An officer said no doubt that those trapped inside must be in a bad situation without food and water.
"But there is hope against hope that they are surviving somehow as the temperature inside the tunnel is about 20-25 degrees Celsius and some oxygen is possibly available to them," the officer said.
Workers at the 480-MW Tapovan-Vishnugad hydel power project of the NTPC got trapped after a surge of water in the Alaknanda river system on Sunday, possibly triggered by an avalanche or glacier break.
Apart from the over 450 personnel from the ITBP, the National Disaster Response Force, the State Disaster Response Force and the army, the family members of those possibly trapped inside have stayed put outside the tunnel in anticipation of some news.