The blast triggered by a build-up of methane gas tore through the tunnel on Monday at the 500-megawatt Teesta Hydroelectric Project in northeastern Sikkim state in the Himalayas.
Authorities have recovered the bodies of 12 workers who died inside the tunnel about 40km south of the state capital Gangtok.
Rescue teams equipped with specialised breathing equipment and night-vision gear were having difficulty entering the tunnel because of the gas and a build-up of water, administrative official Anupama Tamling said on Wednesday.
Gandura Oraon, 38, said he and other workers were eating lunch inside the tunnel when a loud explosion forced them to run for the exit.
"Luckily, we found the vehicle that had delivered lunch to us sometime earlier. We got into it and drove out," he said.
As they escaped, Oraon said, they came across their supervisor and urged him to leave with them as well.
Instead, he headed towards the site of the blast to see what had happened and was later found dead, Oraon said.
The Teesta River basin, where the hydropower project is being built, lies in a seismically active region marked by young, fragile rock formations riddled with fractures and underground pockets that can trap ancient gases.
Fragile mountains, seismic activity and unpredictable underground conditions make tunnel projects especially risky in the region, according to experts.
A February explosion in a coalmine in neighbouring Meghalaya state killed 18 workers.
A tunnel section collapsed in 2023 in northern Uttarakhand state, trapping 41 workers for 17 days before they were rescued.
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