The highest temperature that Jonathan Paul has ever recorded in a London Tube station is about 42 Celsius, or 107.6 Fahrenheit. Fitting air-conditioning units to trains risks heating up the tunnels even more, as warm air from inside carriages gets dumped into the aging tubes. And he’s testing it deep in a chalk quarry to the west of London, near the town of Reading. Today, trains and underground transport networks can be so uncomfortable during heat waves that many passengers avoid using them altogether. Climb 20 meters down a ladder into that chalk quarry near Reading and you will find it.