How did workers get stuck in huge tunnels?

In the area of Nepal impacted by the flash flooding, there are hydropower sites dotted along the Trishuli River. These hydro plants have vast tunnel networks sprawling underground. When the floodwaters pummelled through the mountains and inundated the river, they filled up many of these tunnels with mud. Some workers were lucky enough to get out, but it is estimated more than 150 people are still trapped. Rescue efforts have involved "blasting boulders" to dig them out, with army perso