The deep-level tunnels are buried under London clay - which is lovely to tunnel through, but also a heat insulator. The blog post continued:
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.
That plus the movement from the trains, the engines and the electrical and auxiliary systems add up to one big, hot mess.
However, there is hope in the inventive solutions put forward by the engineer, including the intriguing-sounding "ice cubes experiment", new ventilation shafts and utilising cool ground water to lower the heat in some stations.