The mountain gave warning signs: How the Teesta tunnel disaster unfolded
Twenty-two workers are now confirmed dead. Three others are still feared trapped beneath a Himalayan mountain. Days after a suspected methane gas explosion ripped through
Twenty-two workers are now confirmed dead. Three others are still feared trapped beneath a Himalayan mountain. Days after a suspected methane gas explosion ripped through an under-construction tunnel at NHPC's Teesta Stage VI Hydroelectric Project in South Sikkim's Namchi district, rescue teams continue to battle toxic gases, water ingress and unstable conditions deep underground. But project documents suggest the mountain may have offered warning signs long before the explosion. Read Full Story So, what exactly is the Teesta Stage Six project? And what happened inside the tunnel that turned an ordinary construction shift into a catastrophe? To understand what went wrong, India Today's Open Source Intelligence (OSINT) team analysed official project documents, engineering layouts and satellite imagery. In our visual reconstruction below, we break down the project, the tunnel network, Adit 3 and what led to the explosion. India Today found that while methane can remain trapped within rock formations and fuel underground explosions, NHPC had documented earlier releases just 2 km from the blast site, raising an important question: were the known risks adequately contained? Teesta Stage Six is NHPC's 500 MW run-of-the-river hydroelectric project, located on the Teesta River in South Sikkim's Namchi district. The project's 26.5-metre-high barrage is being built near Sirwani village, close to Singtam, while the underground powerhouse is located downstream near Tarkhola village.
It is the final project in the si project Teesta cascade planned across the river in Sikkim. Unlike conventional dams that create vast reservoirs, a run-of-the-river project diverts part of a river's natural flow through tunnels to generate electricity before releasing the water back into the river downstream. The project consists of four 125 MW generating units, with water carried through two parallel headrace tunnels, each nearly 13.8 kilometres long, to an underground powerhouse. Electricity generated at the project is stepped up to 220 kV before being transmitted to the national power grid through Rangpo. Site of methane releases along the critical tunnel stretch, including one about 2 km from the July 20 blast site. The July 20 accident occurred inside Adit 3, a 347-metre-long access tunnel located between the barrage and the underground powerhouse. The tunnel serves as one of four access passages used by engineers and workers to excavate the main headrace tunnels. Unlike the finished tunnels that will eventually carry water, Adits are temporary construction passages used to transport workers, heavy machinery and construction material deep inside the mountain. The project layout identifies four such Adits, with Adit 3 providing access to one of the most critical excavation faces. According to NHPC, at 1:04 pm on July 20, workers encountered a suspected methane gas pocket trapped within the surrounding rock.
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