Gift link: https://www.wsj.com/world/the-london-tube-feels-like-hell-ef...

> In 2003, then-Mayor of London Ken Livingstone launched a competition with a £100,000 prize for a tunnel-cooling solution, or about $159,500 at the time. Of the 3,400 entries, many simply suggested riders not wear clothes. Other proposals included handing out Popsicles or putting up pictures of snowmen. No practical designs emerged, and the competition was closed in 2005.

The cool thing (pun intended) about this is that the previous clement temperature of the underground was an effectively non-renewable resource. The thermal capacity of the earth around the tunnels is so large that it takes a very long time for it to be changed. Once that capacity was used up, it would also take a very long time to get it back.

There is a direct analogy with primary geothermal energy, which is effectively non-renewable heat mining. Despite common misconception it is not exploiting the average rate of flow of heat up from the deeper Earth.