How expensive is it to build a tunnel? The cost to build a tunnel varies significantly based on its length, location, geology, and purpose. Project costs can range from millions for a small road tunnel to billions for a long underwater passage.

   1. Ballpark costs (typical ranges)

   Regular TBM-driven tunnel (e.g., single running tunnel for road/rail, ~5–8 m dia.): ~$25–60 million per km in many countries; can be higher in difficult ground or with complex systems.

   Urban metro tunnels (tunnel + stations): $150–500+ million per km once you include deep stations, systems and urban mitigation — many metro projects fall in this band.

   Very large, complex or risky projects (long undersea tunnels, huge alpine tunnels, projects in the U.S. with heavy regulations/relocations): hundreds of millions to billions per mile; U.S. urban projects sometimes hit $600M–$2.5B per mile for complicated cases.

   2. Real project examples

   ① Gotthard Base Tunnel (Switzerland): $12B for 57 km (~$210M/km), deepest and longest railway tunnel .

   ② Eurasia Tunnel (Turkey): $1.24B for 14.6 km (~$85M/km), first undersea road tunnel in Istanbul .

   ③ Rogfast Tunnel (Norway): $2B for 26 km (~$77M/km), world’s longest and deepest underwater tunnel (2033 completion) .

   ④ Barcelona Metro Line 9: $3.5B for 41 km (~$85M/km), including TBM and urban challenges .

   ⑤ Channel Tunnel (UK–France): total construction cost in the original program ran into multiple billions (project-level costs ~£4.6–9.5B in 1990s estimates depending on what’s counted).

   These are very rough — a single geological surprise or requirement for extra safety systems can double or triple cost. Use the World Bank / tunnelling guidelines to build a risk-based estimate and include large contingencies.