The tunnels were built to harness the power of Nepal's rushing mountain rivers. Now, they have become the focus of one of the country's most desperate rescue missions.

Scenes from the floods in Nepal and Tibet are catastrophic. But another disaster is unfolding underground and out of sight.

More than 900 workers linked to hydropower projects in flood-ravaged districts near the two countries' border are feared trapped after a cataclysmic deluge of ice, rock, mud and water swept through the region last Wednesday.

Under floodlights and a backdrop of collapsed hillsides, rescuers are excavating around the clock in a bid to reach those trapped underground.

Their condition remains unknown.

But Melbourne-based geologist and engineer Arnold Dix, who has flown to Nepal to provide technical advice to the emergency crews, says any survivors could live for as long as 17 days.

He says some of the underground power stations likely withstood the immense pressure of the flash flooding. However, it remains unclear whether emergency systems activated in time or whether water and debris inundated them.

Professor Dix describes the terrain as a "lunar landscape".

"Everything is just missing," he says, and that is making it difficult to even find the access tunnels for the underground power stations.

For rescuers, each site presents its own challenge.

This map shows the layout of the Trishuli 3B hydro power plant.

As water flows down the river, some is diverted into the plant's intake and through a kilometres-long "headrace tunnel" that runs under the mountains that tower immediately beside the riverbanks.

It then reaches the powerhouse, where the energy of the rushing water is converted into electricity.

"The deluge basically wiped out 27 power stations," Professor Dix says. "Of them, 12 are destroyed, and there's nothing left at the surface.

"We are focusing our efforts on the subterranean spaces that we know will have survived the impact of that debris coming down the valley."

Those efforts are focused on the power station structures themselves, not the 'headrace' tunnels that carry water through the systems.

The Trishuli 3A hydropower project, where Professor Dix is assisting, includes a 33-metre facility that is buried underneath the mountains.

Rooms inside these underground structures, along with supplies, such as water and equipment, could improve the chances of survival for any trapped workers, although dwindling air supplies remain a major concern.

Forty-two workers have been reported missing here, and rescuers are concentrating on locating and entering the access tunnel that leads to the underground powerhouse.

Photos taken during the construction phase of the plant provide a sense of its enormous scale.

The Trishuli River once flowed well below the entrance to this tunnel.

Now, the riverbed itself lies buried beneath flood debris and there's no visible sign of the tunnels at the surface.

"The topography completely changed," Professor Dix says.

"We had to use advanced satellites, technical drawings and vision coming in from the helicopters on the ground to find the exact right locations."

Search and rescue efforts have also been hampered by poor weather, mud, and the loss of critical infrastructure like roads and bridges.

Despite the uncertainty, there is hope some workers may have reached what engineers refer to as the "Goldilocks zone".

"The Goldilocks zone, where we think people might be, is right up at the top of the underground powerhouse in the area between where the tunnels enter the power station and the roof," Professor Dix says.

This cross-section diagram of the powerhouse shows what Professor Dix is talking about — if rescuers are to find any survivors, they are likely to be in the highest section of the 11-storey underground structure, above the power cubicle.

"So that's where we're hoping that people [are] who've survived the initial explosion — and that's effectively what this is. It's not a flood, it's an explosion.

"There's plenty of air there in the Goldilocks zone, and it's a large space. If there was any place you were going to survive, it would be there."

Jenjen Lama, a mountain guide involved in the search-and-rescue operations, says a similar operation is underway at the Chilime Hydropower Station and has provided the ABC with a diagram of where engineers believe survivors may be located.

The cross-section image shows a 250-metre tunnel leading downhill before connecting with the powerhouse, which includes a service bay, control room and the hydropower transformer.

Again, experts believe the area at the top of the underground powerhouse is where survivors could be located.

But reaching these areas presents its own challenges.

Professor Dix says it will likely require rescuers to drill vertically from the surface and then break through reinforced concrete at the top of the powerhouse.

He is working on a strategy to do exactly that.

However, one of the major concerns for rescuers is the fact that mud and water found at the tunnel entrance could indicate flooding deeper inside the facility.

"If the water's there, that probably means we've got water in the powerhouse up above the level we want," Professor Dix says.

A rescue beyond the textbook

Professor Dix has advised on major underground rescue operations around the world. Three years ago, he was hailed a Himalayan hero after helping rescue 41 workers who had been trapped for 17 days after a tunnel collapsed in India.

But he says the conditions in Nepal are unlike anything he has encountered.

"There are boulders bigger than houses — I've never seen that before in my life," Professor Dix says. "It's just an extraordinary operation."

Although helicopters are ferrying personnel and supplies into some of the hardest-hit areas, payload restrictions have constrained what equipment can be brought in.

"We can get in any machinery we want as long as it's less than three tonnes," Professor Dix says. "So, in other words, we can't have any."

"We're using hands and shovels.

"It's well outside any textbook rescue."

Filmmaker and drone pilot Manish Maharjan, who has been volunteering with the Nepalese army, says some tunnels are "filled with mud like toothpaste in a tube".

At one hydropower project, he used a drone to search to the end of a tunnel but found no sign of life.

But as long as there remains a chance that hydropower workers alive in pockets of air underground, the search continues.

In a social media post, Nepalese Prime Minister Balendra Shah said 279 workers had been rescued so far.

Arnold Dix is an optimist, though he admits the situation is dire.

"If there are people alive out there, we are doing our best to find them."

Credits:

- Satellite images: Vantor

- Reporting: Hanan Dervisevic

- Design: Jarrod Fankhauser

- Production: Jessica Riga

- Development: Joshua Byrd

- Editing: Matt Liddy