Drone pilot Manish Maharjan has helped rescue teams in Nepal.

They are looking for people who went missing after last week's floods.

He sometimes used speakers on the drones.

He called out or played music for people stuck on hillsides.

But the search for hundreds of workers is a different challenge.

These workers are believed to be trapped in power plant tunnels.

Maharjan said the tunnels were full of mud.

It was like toothpaste in a tube, with no empty space inside.

He joined the Nepal Army's rescue efforts one day after the Aug. 26 flash floods.

Those floods killed more than 1,000 people.

They left thousands missing in Nepal and Tibet.

The floods happened because a glacier collapsed.

They damaged roads, buildings, bridges, and hydropower plants.

At least 900 workers are missing from 12 hydropower projects.

Mud and debris buried these projects.

About 500 of them are believed to be trapped inside tunnels.

Some workers managed to call for help when the disaster struck.

Mohan Kumar Dangi said this.

He is the president of the Independent Power Producers' Association.

It is not clear if rescuers have had contact with them since.

Maharjan has been flying thermal drones in the tunnels.

He said searches are hard because it is difficult to get in and there is no empty space.

He can only fly drones in long, straight tunnels.

Bends or zigzags make it much harder.

At the Rasuwagadhi hydropower project, he sent a drone to the end of a tunnel.

He found no sign of life.

He said it was full of mud and debris.

He could only imagine the force of the flood.