Nepal's maze of hydropower tunnels that turned into traps

Nepal’s tunnel maze that trapped hydropower workers

After a massive chunk of the Langtang Lirung glacier collapsed in Nepal on August 26, the resulting landslide caused widespread devastation downstream, killing more than 1,400 so far while about 5,700 remain missing. One of the biggest challenges for rescue teams has been navigating the hidden architecture beneath Nepal’s hydropower stations: a network of tunnels deep in the Himalayas that authorities believe trapped workers stationed inside them. The disaster washed away around a dozen hydropower projects along the Trishuli river, burying some under metres of mud, rock, timbeand debris. Efforts to clear the wreckage has required weeks of drilling, controlled blasting and heavy equipment. So far, only a few workers have been rescued and more than two dozen bodies have been recovered.

Floods, landslides overwhelmed dozens of hydropower projects Some of the hydropower projects affected along the Trishuli river were as far as 75 km or so from the site of the glacier collapse. Glacier collapse NEPAL Extent of mud flow Rasuwa Bhotekoshi Langtang Khola Rasuwagadhi CHINA Upper Trishuli 1 Chilime Upper Trishuli 3A NEPAL Mailung Khola Upper Trishuli 3B Trishuli 5 km Devighat North Glacier collapse NEPAL Extent of mud flow Rasuwa Bhotekoshi Langtang Khola Rasuwagadhi Chilime CHINA NEPAL Upper Trishuli 1 Upper Trishuli 3A Mailung Khola Upper Trishuli 3B Trishuli Devighat 5 km North Glacier collapse NEPAL Extent of mud flow Rasuwa Bhotekoshi Langtang Khola Rasuwagadhi Chilime Upper Trishuli 1 CHINA NEPAL Upper Trishuli 3A Mailung Khola Upper Trishuli 3B Trishuli Devighat 5 km North Glacier collapse NEPAL Extent of mud flow Langtang Khola Rasuwa Bhotekoshi Upper Trishuli 1 Upper Trishuli 3A Chilime Rasuwagadhi Trishuli CHINA NEPAL Upper Trishuli 3B Devighat Mailung Khola 5 km North North NEPAL Extent of mud flow Rasuwa Bhotekoshi Langtang Khola Chilime Rasuwagadhi Upper Trishuli 1 Upper Trishuli 3B Trishuli Devighat Upper Trishuli 3A CHINA NEPAL Mailung Khola 5 km Glacier collapse Regional map of Nepal showing the glacier collapse site, extent of mud flow along the river, and locations of multiple hydropower projects.

Finding the vast expanse of hydropower infrastructure beneath the mountains made rescue efforts especially difficult. The tunnel entrances, along with the roads connecting them, were all buried under mud, making it difficult to determine where to start digging. Many hydropower projects have massive underground infrastructure A typical run-of-the-river hydropower project consists of a mix of above-ground and extensive underground infrastructure. Let’s look at the Rasuwagadhi hydropower project, the first one hit by the disaster, where authorities believe dozens of workers were trapped.

Overground infrastructure Reservoir and diversion weir Trishuli river Timure village Access road Main access points and tailrace outlet N Overground infrastructure Trishuli river Reservoir and diversion weir Timure village Access roads Main access points and tailrace outlet North Overground infrastructure Trishuli river Reservoir and diversion weir Timure village Access roads Main access points and tailrace outlet North Access roads Overground infrastructure Trishuli river Reservoir and diversion weir Timure village Main access points and tailrace outlet North North Overground infrastructure Reservoir and diversion weir Trishuli river Timure village Access road Main access points and tailrace outlet N Overground infrastructure Trishuli river Reservoir and diversion weir Timure village Access roads Main access points and tailrace outlet North Overground infrastructure Trishuli river Reservoir and diversion weir Timure village Access roads Main access points and tailrace outlet North Access roads Overground infrastructure Trishuli river Reservoir and diversion weir Timure village Main access points and tailrace outlet North North Tunnel inlet portal Underground desanding basin N Tunnel inlet portal Underground desanding basin North Tunnel inlet portal Underground desanding basin North Tunnel inlet portal Underground desanding basin North Underground power house Headrace tunnel Tailrace outlet Tailrace tunnel Surge shaft Valve chamber N Surge shaft Headrace tunnel Tailrace outlet Tailrace tunnel Underground power house Valve chamber North Surge shaft Headrace tunnel Tailrace outlet Tailrace tunnel Underground power house Valve chamber North Surge shaft Headrace tunnel Tailrace outlet Tailrace tunnel Underground power house Valve chamber North Tunnels for aeration, construction and access to valve chamber Desander operation tunnel Desanding flushing / adit tunnel Construction adit Construction adit Main access tunnel to powerhouse Penstock construction adit Transformer cavern access N Transformer cavern access Desanding flushing / adit tunnel Construction adit Desander operation tunnel Main access tunnel to powerhouse Construction adit Penstock construction adit Tunnels for aeration, construction and access to valve chamber North Transformer cavern access Desanding flushing / adit tunnel Construction adit Desander operation tunnel Construction adit Main access tunnel to powerhouse Penstock construction adit Tunnels for aeration, construction and access to valve chamber North Access to transformer cavern Desanding flushing / adit tunnel Desander operation tunnel Construction adit Construction adit Main access tunnel to powerhouse Penstock construction adit Tunnels for aeration, construction and access to valve chamber North Extent of mud flow N Extent of mud flow North Extent of mud flow North Extent of mud flow North Extent of mud flow N Extent of mud flow North Extent of mud flow North Extent of mud flow North A map shows the locations of overground infrastructure of the Rasuwagadhi hydropower power project, including reservoir and diversion weir near the inlet, and main access points and tailrace outlet, along the Trishuli river in Nepal. The infrastructure above ground mainly consists of a reservoir and a diversion weir. Water is diverted into underground tunnels, before reappearing at an outlet some 4.5 km downstream. The same map is repeated with an explanatory text. Under the mountain, the water first goes through a desander, which removes sediment, before it is channelled toward the power turbines. The same map now shows the tunnel inlet portal and underground desanding basin. From there, the water heads into a 4-km-long headrace tunnel that carries it to a huge multi-storey underground powerhouse complex, where electricity is generated and fed into a transmission line to carry power to the grid. The same map now shows the alignment of headrace and tailrace tunnels, underground power house, surge shaft and valve chamber. Connecting this massive underground infrastructure to the ground is a network of tunnels, some of which were used only temporarily, such as during the construction of the plant. Others give workers access to the powerhouse and other infrastructure or provide for aeration and desanding. The same map now highlights a network of tunnels along the route, connecting various parts of the underground infrastructure. This is the underground infrastructure – tunnels and the powerhouse – where workers were trapped during the disaster when tunnel entrances and access roads were buried under heavy mud. The same map now shows the extent of mud flow along the river. The same map is now repeated.

A closer look at the area around Rasuwagadhi’s powerhouse shows a complex and interconnected network of tunnels with independent access points and different uses, going several hundred metres inside the mountainside.

Tunnel network around Rasuwagadhi project’s powerhouse area Transformer room Powerhouse Adit Concrete wall Aeration tunnel Adit Access to tranformer room Penstock construction adit 160 m Mountainside Access tunnel to valve chamber Main access tunnel Headrace tunnel Aeration tunnel Access tunnel to valve chamber Mountainside Concrete wall Penstock construction adit Headrace tunnel Powerhouse Main access tunnel Adit Access to tranformer room Transformer room 160 m Towards tailrace tunnel Adit Aeration tunnel Surge shaft Access tunnel to valve chamber Valve chamber Mountainside Concrete wall Penstock construction adit Headrace tunnel Powerhouse Main access tunnel Construction adit Access to tranformer room Transformer room 160 m Towards tailrace tunnel Adit Aeration tunnel Surge shaft Access tunnel to valve chamber Valve chamber Mountainside Concrete wall Penstock construction adit Headrace tunnel Powerhouse Main access tunnel Construction adit Access to tranformer room Transformer room 160 m Towards tailrace tunnel Adit Towards tailrace tunnel Schematic diagram of the tunnel network around Rasuwagadhi project’s powerhouse area, showing the surge shaft, valve chamber, headrace tunnel, powerhouse, transformer cavern, main access tunnel, etc.

These tunnels vary in size and structure, depending on their purpose. Several tunnels were packed or mostly filled with debris from the mud up to hundreds of metres from their entrances.

Tunnels vary by size and purpose An example of two tunnels that are part of the vast network of tunnels at the Rasuwagadhi power project Exhaust air/smoke for transformer cavern Fresh air duct Exhaust Main access tunnel Access to transformer cavern Exhaust air/smoke for main powerhouse Exhaust air/smoke for transformer cavern Fresh air duct Exhaust Main access tunnel Access to transformer cavern Exhaust air/smoke for main powerhouse Exhaust air/smoke for transformer cavern Fresh air duct Exhaust Main access tunnel Access to transformer cavern Exhaust air/smoke for main powerhouse Fresh air duct Exhaust air/smoke for main powerhouse Exhaust Main access tunnel Access to transformer cavern Exhaust air/smoke for transformer cavern Cross-section diagrams comparing two tunnel types at the Rasuwagadhi project — the main access tunnel and access to the underground transformer room — showing ventilation ducts and vehicle size for scale.

Several tunnel entrances were blocked and covered by the landslide, complicating rescue efforts to determine the alignment of the tunnels, including at the Upper Trishuli 3A power project, where two workers were pulled out on September 4, nine days after the disaster.

Rescue team members work to enter the tunnel at the Upper Trishuli 3A Hydropower Station as they search for survivors and retrieve bodies, following deadly flash floods and a mudslides in Rasuwa, Nepal, September 3, 2026. REUTERS/Navesh Chitrakar

Serious excavation work there didn’t begin until three days after the disaster, when a breakthrough from a borehole drilled from above confirmed the tunnel’s alignment, giving crews a fixed point to dig toward. Highlighting the difficulties of the task, it took rescuers nearly six days to reach the tunnel’s entrance buried deep in the debris. All tunnels at the site were buried, some under 15 to 30 metres of debris, and the workers were ultimately pulled out from a tunnel used to carry cables. A Reuters analysis shows the tunnel entrance was about 29 metres above the river bed and the landslide had buried it under another roughly 20 metres of mud.

Landslide overwhelmed tunnel entrances at Upper Trishuli 3A Cable tunnel entry 29 meters above river bed After disaster Cable tunnel entry Mud flow 50 meters above river bed Before disaster Cable tunnel entry 29 meters above river bed After disaster Mud flow Cable tunnel entry 50 meters above river bed Before disaster Cable tunnel entry Cable tunnel entry 50 meters above river bed 29 meters above river bed After disaster Mud flow Before disaster Cable tunnel entry Cable tunnel entry 50 meters above river bed 29 meters above river bed After disaster Mud flow Before disaster Satellite image comparison of the Upper Trishuli 3A cable tunnel entrance before and after the disaster, showing mud flow rising up to 50 metres above the riverbed. Satellite imagery: Google © 2026 Airbus (before disaster), and Planet Labs (after disaster).

Engineering drawings seen by Reuters show one access tunnel to the station ran for more than 180 metres to the underground powerhouse — but rescuers believed it was flooded and offered no hope of finding survivors. So they turned their attention to the shorter cable-duct tunnel that sloped upward and was less exposed to the floodwaters. That is where the two rescued workers had taken shelter, just 160 metres from the exit. The pair survived in an air-filled chamber, said lawmaker Shri Ram Neupane, a veteran tunnel engineer who helped lead the rescue effort.

Survivors rescued from Upper Trishuli 3A power project Survivors pulled out from this area Powerhouse Access tunnel Cable duct tunnel Tailrace tunnel Extent of mud flow (outer surface) 25 m Headrace tunnel Survivors pulled out from this area Underground powerhouse Access tunnel Cable duct tunnel Tailrace tunnel Extent of mud flow (outer surface) 25 m Headrace tunnel Survivors pulled out from this area Underground powerhouse Access tunnel Cable duct tunnel Tailrace tunnel Extent of mud flow (outer surface) 25 m Headrace tunnel Satellite map showing rescue location at Upper Trishuli 3A power project, with labelled headrace tunnel, underground powerhouse, access tunnel, cable duct tunnel, and tailrace tunnel, alongside the extent of mud flow. Satellite imagery: Google © 2026 Airbus

One of the survivors, mechanical foreman Sanjay Sah, said he worked in the powerhouse’s control room and had sped from floor to floor, warning colleagues of the oncoming deluge. “Before the flood, I knew every part of it and could have gone deep inside without difficulty. But after the flood, the entire structure had been transformed,” he said. “Equipment had been swept away, boxes were scattered everywhere, walls had collapsed and debris had created mounds and pits. My familiarity with the tunnel was no longer useful.”

Survivor Kabir Maharjan is carried by members of a search and rescue team after they rescued two people alive who had been trapped inside a tunnel at the Trishuli-3A Hydropower Project following devastating floods and mudslides that struck the Bhotekoshi River in Rasuwa, Nepal, September 4, 2026. Nepal Army/Handout via REUTERS

Rescuers later identified an escape ladder that could have saved as many as two-thirds of those inside had they known about it, said Suraj Dahal, an engineer at the Nepal Electricity Authority, who led parts of the rescue effort. Many could have escaped, he said, if they had the muscle memory of the escape route. “They had time to escape, but did not know where to go,” he said.

Jan Jiwan Thakur, a civil engineer at Upper Trishuli 1, said that when the siren went off, he and nearly 100 colleagues ran uphill on instinct from past safety drills. However, he noted they were trained for emergencies in general, not flash floods specifically. “Ideally, we should have gone to the safer area inside the tunnel, but we weren’t aware of it,” he said. Dhan Bahadur Tamang, a contract driver, said he had never received any training. He escaped the tunnel during the deluge only because he knew the way out. “The contractor only gives us safety helmets and shoes in the name of training,” he said. “I was lucky to come out alive, but many others lost their lives.”

Rescuers used various methods to try to get to survivors, including the use of excavators to move boulders from the mouth of the tunnels, drilling, and controlled blasting. “The flood has altered the terrain around the tunnels and changed their structure, while mud, rocks, soil and other debris had accumulated inside,” Nepal ​army spokesperson Raja Ram Basnet told Reuters. “This is not a normal closure of tunnels.”