By Saurabh Sharma and Joseph CampbellTARKESHWAR, Nepal, Sept 3 (Reuters) - Dhan Bahadur Tamang was at work in a hydropower project near Nepal's border with China when he sensed a sudden air pressure change in the underground tunnel at the Upper Trishuli 1 hydropower project.Under the industrial floodlights, dust started to rise, the 32-year-old recalled: "Something was coming."

Tunnel expert Arnold Dix says large underground areas may have sheltered workers from the devastation, as rescue work continues in hydropower sites.

Rescue efforts have increasingly centred on hydroelectric project tunnels along banks of Trishuli river

At least 557 workers are missing from a hydropower project along the Trishuli River, with about 115 still believed to be trapped.