[NamTaf]

Reading a bit more into this, I'm still not sure whether the tunnels are actually continuing to meaningfully increase absent external changes to the system. Is it at a steady-state heat flux between the inputs (passenger, vehicles, infra) and output (into the surrounding earth), or is the surrounding earth still not effectively saturated?

I know heat inside the tunnels fluctuates with time of year, but I can't tell if year-on-year the tunnels are getting e.g. 1xx% hotter and, critically, what the causes are (more tph will obviously, global warming will, more pax will).