What caused the NHPC tunnel disaster in Sikkim's Namchi?

Story so far: Toxic gases, generated by a sudden explosive release of methane trapped or stored in rocks on July 20, 2026, claimed the lives of 15 workers (till reporting time) at NHPC’s 500 MW Teesta’s Stage VI Hydropower Project in Sikkim under construction Race Tunnel (HRT) Five others remained trapped around 1.5 km inside the tunnel, underscoring the dangers of infrastructure development across the Himalayan landscape.

How did the tragedy happen?

The NHPC said a thorough investigation would be conducted to find out 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 due to the sudden release of what is believed to be methane trapped in the rocks, resulting in an explosion and partial damage to the tunnel. This suggests that workers encountered such rocks while drilling or building the HRT, a critical part of the hydroelectric plant for transporting huge volumes of water from the primary intake in the reservoir to the turbine for power generation.

Also Read: Silkyara Tunnel Collapse | Trapped in the Himalayas

NHPC officials said surveys of tunnel projects in India, particularly in the fragile Himalayan belt, are conducted across planning, construction and maintenance phases to ensure structural stability, worker safety and environmental compliance. These usually include comprehensive topographical, geotechnical, geophysical, hydrological, resistance and utility tests. However, no test can detect the presence of colorless, odorless methane, an extremely flammable gas that can explode when mixed with air in a certain proportion and collide with an ignition source, or sparks during drilling in the case of the Teesta-VI tunnel.

How is methane captured in hilly terrain?

Methane deposits usually occur either as natural gas trapped in porous rocks or as localized gas pockets in young, brittle geological formations. They are often found in the Himalayan belt or other mountainous regions with active or historic coal beds where ancient organic matter has been decomposing for millions of years. Hilly depressions or valleys often trap moisture and create oxygen-deprived (anaerobic) waterlogged soils where microscopic organisms digest organic matter and release methane. Improperly managed or open waste dumps in hilly terrain also decompose in anaerobic layers and produce large amounts of landfill gas.

In addition, hilly and mountainous areas are often located near fault lines or areas with underlying coal, gas, or geothermal resources, with natural fissures allowing underground methane to migrate upward. There are no coal fields in Sikkim, but the state is located in the Rangit tectonic window zone and the Geological Survey of India has identified coal reserves of around 1.40 lakh tonnes in South Sikkim, mainly in the Namchi area where the NHPC tunnel incident occurred.

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

Were other projects in Sikkim similarly affected?

Due to its young geological age, active tectonic instability and steep terrain prone to erosion, the Himalayas are fragile. The mountains are constantly rising and shifting due to the collision of the Indian and Eurasian plates, making the region very prone to earthquakes. Heavy blasting and tunneling on highways, dams and railways contribute to geological instability, leading to frequent landslides and mudslides. Heavy tourism, unregulated urban expansion, poor waste management and deforestation have exacerbated these problems.

Sikkim has also suffered from extreme weather events such as cloudbursts, rapid melting of glaciers and expanding glacial lakes attributed to climate change. Hydro projects in the state have suffered as a result of several of these factors.

In October 2023, a glacial lake burst flood killed more than 90 people and destroyed the 1200 MW Sikkim Urja Limited dam at Chungthang in Mangan district. The disaster also damaged NHPC’s 510 MW Teesta-V Hydropower Project, which was revived three years later. A landslide 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 a suspected methane leak.

Was tunneling in India prone to disasters?

Tunneling is inherently risky, especially in the fragile Himalayan system and its easternmost extension into the Mizoram hills. Some of the incidents involved NHPC hydro projects. In May 2021, a massive section of a diversion tunnel under construction for the 800 MW Parbati-II project in Himachal Pradesh collapsed, killing four workers. A portion of the roof of a tunnel at the 2000 MW Subansiri Lower Hydroelectric Project on the Arunachal Pradesh-Assam border collapsed in June 2022, killing a worker.

A similar collapse at the project site killed another worker in July 2024. Torrential rains caused a massive landslide at the NHPC Dhauliganga Hydroelectric Project in Pithoragarh, Uttarakhand in August 2025. Fortunately, all 19 trapped NHPC employees were rescued. Non-NHPC tunnel disasters include the double-tubed 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 in a collapsed section of the under-construction Silkarya Bend-Barkot tunnel in November 2023 were rescued. The National Highways and Infrastructure Development Corporation Limited has constructed this tunnel through contractors in Uttarakhand.

Another notable disaster occurred when the Srisailam Left Bank Canal Tunnel collapsed in February 2025, trapping eight engineers and workers. The bodies of two persons were recovered while the others were presumed dead. Manipur experienced its worst disaster in June 2022 when 61 people died after a massive landslide hit the construction site of Tupul railway station which is integrated with several tunnels.

The North East Frontier Railway is carrying out work at this railway station under the 111 km long Jiribam-Imphal project. Railways, NHAI, NHIDCL and NHPC officials say the terrain of the Northeast region warrants tunnels that will fulfill New Delhi’s ambition to provide connectivity to the last villages or habitations along the international border.

Published – 22 Jul 2026 14:59 IST