This photo provided by the Nepal Army, shows Nepal army soldiers using a drilling machine during a rescue operation at the site of a hydropower project in Upper Trishuli, Nepal, Sunday, Aug. 30, 2026. (Nepal Army via AP)Following last week’s devastating flash floods and landslides in northern Nepal, the country’s National Disaster Risk Reduction and Management Authority said Monday (August 31) that 903 people have died and 4,247 remain missing so far, including 933 hydropower workers and 592 foreign nationals.According to a CNN report, these workers are missing at various hydropower projects that run along the Trishuli river, flowing through multiple districts. Rescue crews and foreign specialists are racing to find them feared to be buried under thick mud inside hydropower tunnels, the report said.Although it is unclear how many of these workers are still alive, ongoing search and rescue efforts in these tunnels have faced significant hurdles in the form of poor weather, mud and debris, and the loss of critical infrastructure such as roads, bridges, and settlements.Here’s what to know about the challenges that arise during rescue operations in hydropower tunnels.Mud and debrisAccording to a list compiled by Nepal’s Independent Power Producers’ Association last week, the workers were missing at 11 different hydropower projects affected by the floods. Each of these power plants presents its own challenges to rescue teams.Also in Explained | Nepal floods: The red flags in dam-building in the HimalayasRescue workers have to dig through thick mud and debris, bore through tunnel doors or walls, and when necessary, use explosives to blast them open. Even if they find a way in, the tunnels are often filled with sludge brought in by the floods. For instance, at the Trishuli 3A hydropower plant, army personnel were trying to breach a tunnel but had managed to enter by only about seven yards, a Nepal Army spokesperson told The New York Times.Australia-based geologist Arnold Dix, who has advised rescue teams at Trishuli 3A, told CNN that it wasn’t clear how much of the tunnel had been filled with mud or water. Dix listed other risk factors, such as the failure of emergency shutdown systems could lead to the turbine room getting filled with water and potentially drowning anyone inside. According to him, this water could be released owing to drilling by the crews — a potential risk for rescue workers.Story continues below this adSome of these hydropower projects have tunnels that could be several kilometers long — this means that workers could be some distance away from the tunnel entrance or the portion that rescue teams are attempting to breach. For instance, the tunnel that leads to the underground powerhouse at Trishuli 3A is more than 4 km long.Bridging this distance becomes harder especially in case of poor weather. Reports from Trishuli 3A pointed to overnight rain causing river levels to rise, thereby obstructing rescue operations. Such rainfall can also flood areas already cleared by rescue teams, besides triggering fresh landslides and exacerbating the risk that damaged roads present in times of disaster.NewsletterFollow our daily newsletter so you never miss anything important. On Wednesday, we answer readers' questions.SubscribeThis also disrupts helicopter and drone operations, which are critical in terms of access to remote sites and movement of rescue personnel and equipment. A mountain guide involved in rescue operations at the Chilime hydropower plant told CNN that damaged infrastructure had made it hard for helicopters to land there. Rescue teams have also deployed thermal cameras on drones to search for survivors — bad weather could halt operations until conditions improve.Air supply in the tunnels could be a major problem. On Saturday (August 29), Nepal’s Home Minister Sudan Gurung, while announcing on social media that rescue teams had reached the mouth of Trishuli 3A, said that they were “supplying air through that small hole”.Story continues below this adNepal’s government said last week that teams from India and China would provide technical assistance in the rescue operations. In recent years, both countries have made significant investments in hydropower projects across rivers in the Himalayan region in an attempt to harness their immense energy potential.