A structural engineer has gone full bore geek on the problem that is cooling Londons tube system.
I could have done without learning this:
In fact, when the early tube tunnels were dug, they were so cool down there that the cool tube was seen as a respite from the summer heat on the surface. Why suffer on a bus in the heat when there’s a cool tube to take instead, said the marketing men.
Over the years, the heat from the trains soaked into the clay to the point where it can no longer absorb any more heat. Tunnels that were a mere 14 degrees Celsius in the 1900s can now have air temperatures as high as 30 degrees Celsius on parts of the tube network.