What caused the NHPC tunnel disaster in Sikkim’s Namchi? | Explained

The story so far: Toxic gases, generated by a sudden explosive release of suspected methane trapped or embedded in rocks on July 20, 2026, claimed the lives of 20 workers (till the time of reporting) in an under-construction Head Race Tunnel (HRT) of the NHPC’s 500 MW Teesta Stage VI Hydropower Project at Samardung in Sikkim’s Namchi district. Five others remained trapped about 1.5 km inside the tunnel, underlining the hazards of infrastructure development across the Himalayan landscape.

How did the tragedy happen?

The NHPC has said a thorough investigation will be conducted to ascertain the cause of the incident. The Sikkim government has also announced the constitution of a high-level committee for the same purpose. Preliminary reports indicate the formation of toxic gases in the HRT owing to the sudden release of suspected methane trapped in the rocks, resulting in an explosion and partial damage to the tunnel. This indicates that the workers hit such rocks while drilling or constructing the HRT, a critical component of a hydropower plant for transporting massive volumes of water from the primary intake at a reservoir to the power generation turbine.

Also read: Silkyara tunnel collapse | Trapped in the Himalayas

NHPC officials said that surveys for tunnel projects in India, especially in the fragile Himalayan belt, are conducted across planning, construction, and maintenance phases to ensure structural stability, worker safety, and environmental compliance. These typically involve comprehensive topographical, geotechnical, geophysical, hydrological, resistivity, and utility tests. However, no test can detect the presence of the colourless and odourless methane, an extremely flammable gas, which can explode after mixing with air in a specific ratio and encountering an ignition source, possibly sparks during drilling in the case of the Teesta-VI tunnel.

How is methane trapped in hilly terrains?

Methane deposits usually occur either as natural gas trapped in porous rock or localised gas pockets in young, fragile geological formations. These are frequently encountered in the Himalayan belt or other mountainous regions with active or historical coal-bearing strata, where ancient organic matter has decomposed over millions of years. Hilly depressions or valleys frequently trap moisture, creating oxygen-deprived (anaerobic) waterlogged soils where microscopic organisms digest organic matter and emit methane. Improperly managed or open waste dumps in hilly terrain also decompose in anaerobic layers, producing large amounts of landfill gas.

Moreover, hilly and mountainous regions are often located near fault lines or areas with underlying coal, gas, or geothermal reserves, with natural fissures allowing the migration of underground methane upwards. There are no coalfields in Sikkim, but the State is in the Rangit Tectonic Window zone, and Geological Survey of India studies have identified coal reserves of approximately 1.40 lakh tonnes in southern Sikkim, primarily the Namchi area where the NHPC tunnel incident occurred.

According to the State’s Mines and Geology Department, a regional-level coal exploration project commenced in the district in December 2025 under a Government of India programme. The coal in Namchi was analysed to contain 53.40% fixed carbon, 39.5% ash, 3.4% volatile matter, and 3.7% moisture.

Have other projects in Sikkim been similarly affected?

A young geological age, active tectonic instability, and steep, erosion-prone terrain make the Himalayas fragile. The mountains are constantly rising and shifting due to the collision of the Indian and Eurasian plates, making the region highly prone to earthquakes. Heavy blasting and tunnelling for highways, dams, and railways add to the geological instability, leading to frequent landslides and mudslides. Heavy tourism, unregulated urban expansion, poor waste management, and deforestation have compounded the problems.

Sikkim has also suffered from extreme weather events such as cloudbursts, rapidly melting glaciers, and expanding glacial lakes attributed to climate change. Hydropower projects in the State have suffered owing to several of these factors.

A Glacial Lake Outburst Flood in October 2023 killed more than 90 people and destroyed Sikkim Urja Limited’s 1,200 MW dam at Chungthang in Mangan district. The disaster also damaged NHPC’s 510 MW Teesta-V Hydropower Project, which was restored three years later. A landslip damaged some houses and a building of this project in August 2024. The Teesta-VI tunnel disaster is the first recorded incident in the State involving an explosion due to suspected methane gas release.

Has tunnelling been disaster-prone in India?

Tunnelling is inherently risky, particularly in the fragile Himalayan system and its easternmost extension up to the hills of Mizoram. Some of the incidents involved NHPC hydropower projects. A massive section of an under-construction diversion tunnel for the 800 MW Parbati-II Project in Himachal Pradesh collapsed in May 2021, killing four labourers. A section of a tunnel roof at the 2,000 MW Subansiri Lower Hydroelectric Project on the Arunachal Pradesh-Assam border collapsed in June 2022, killing a worker.

A similar cave-in at the project site killed another worker in July 2024. Torrential rains triggered a massive landslip at the NHPC’s Dhauliganga Hydroelectric Project in Uttarakhand’s Pithoragarh in August 2025. Fortunately, all 19 trapped NHPC employees were rescued. The non-NHPC tunnel disasters include the twin-tube Khooni Nallah (by private firms for the National Highways Authority of India) on the Jammu-Srinagar highway in May 2022.

While 10 workers were killed in this incident, 41 workers trapped inside a caved-in section of the under-construction Silkarya Bend-Barkot tunnel in November 2023 were rescued. The National Highways and Infrastructure Development Corporation Limited executed this tunnel through contractors in Uttarakhand.

Another notable disaster occurred when a tunnel of the Srisailam Left Bank Canal collapsed in February 2025, trapping eight engineers and labourers. The bodies of two persons were recovered while the others were presumed dead. Manipur experienced the biggest disaster in June 2022 when 61 people died after a massive landslip hit the construction site of the Tupul Railway Station, which is integrated with multiple tunnels.

The Northeast Frontier Railway is executing the work on this railway station as part of the 111-km-long Jiribam-Imphal project. Railway, NHAI, NHIDCL, and NHPC officials say the northeastern region’s terrain warrants tunnels to fulfil New Delhi’s ambition to provide connectivity to the last villages or habitations along the international borders.