Road or Tunnel How Engineers Choose Routes Through Nepal’s Himalayas

Should Nepal keep its highways on the mountainside — or tunnel straight through the Himalayas? There is no universal answer. A surface highway can remain practical where slopes, drainage, rockfall and river-side hazards can be controlled. A tunnel can bypass difficult surface terrain — but it replaces those problems with another engineering world: fractured rock, groundwater, excavation support, ventilation, emergency escape and decades of underground operation. This documentary explores how engineers make that choice in Nepal. We begin near Nagdhunga, where a localized section of the Prithvi Highway at Pataleban sank on September 2, 2026. Traffic was halted, and eligible small vehicles could be diverted through the new Nagdhunga Tunnel. The contrast was striking: the surface road had a problem, while the underground route provided an alternative. But that does not mean every dangerous mountain road should become a tunnel. On the 33-kilometer Narayanghat–Mugling corridor, engineers instead strengthened the surface highway. More than 100 slopes received protection works using methods that included drainage and bioengineering, while 23 more complex slopes received combinations of stronger site-specific treatments. Nagdhunga represents another strategy. Its approximately 2.68-kilometer road tunnel moves part of the highway beneath difficult terrain, while also requiring ventilation, lighting, communications, emergency systems and a parallel evacuation system. Then there is Siddhababa, where the engineering solution is not simply road or tunnel. The project combines a roughly 1.1-kilometer main tunnel with surface-road upgrading, a 780-meter rock shed and additional bypass-tunnel elements. And on the Kathmandu–Terai Fast Track, roads, bridges and tunnels form parts of a larger corridor, while additional tunnel locations have been considered through design review. The deeper question is not which technology is “better.” It is: What is the dominant hazard? Can the surface