The highest temperature that Jonathan Paul has ever recorded in a London Tube station is about 42 Celsius, or 107.6 Fahrenheit. Paul, a researcher at Royal Holloway, University of London, uses his thermometer-equipped smartphone to take such readings. 42C is the kind of heat that would send someone running to the nearest air-conditioned building. Underground, though, they can’t. There’s nothing but stifling tunnels and screeching trains down here.
The Tube network runs through thick clay, and that dense material has been soaking up heat generated by trains since the tunnels were first dug, in some cases more than 100 years ago. 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.
But Paul has an idea to cool the tunnels themselves. “Water, as a refrigerant, can hold huge amounts of heat,” he says. “It’s everywhere beneath London.” He’s working on a technology that would use groundwater at roughly 10 Celsius to ferry excessive heat away from underground stations. And he’s testing it deep in a chalk quarry to the west of London, near the town of Reading.
Trains, being metal tubes packed with people, are hard to cool down. But as summers get hotter due to climate change, ensuring public transport remains comfortable and safe is becoming ever more important. It’s a global issue. Train riders in Japan and Morocco have complained about insufficient air-conditioning during heat waves this year, and a 2023 study reported train carriage temperatures as high as 47 degrees Celsius in India.
Paul, for one, has witnessed the effects of overheating on commuters using overground trains. “I’ve seen four people faint this summer,” he says.
The first air-conditioned trains date back roughly a century. One 1933 article put it like this: “Until now, every one has dreaded a railway journey in summer.” If only they could have seen the future. Today, trains and underground transport networks can be so uncomfortable during heat waves that many passengers avoid using them altogether.
Paul and his colleagues believe their solution will work. Climb 20 meters down a ladder into that chalk quarry near Reading and you will find it. There are multiple galleries of varying sizes carved into the chalk, separated by doors. “We’re trying to simulate real-life conditions in the Tube,” says Paul—though things down in the quarry are a little more drab. “It’s very dark, it’s quite dingy.”
In 2022, he and a colleague published a paper that described how water from subterranean rivers or aquifers could be pumped into heat exchangers attached to the ceiling above subway platforms. Hot air sucked into these exchangers would pass some of its warmth to the water, allowing cool air to blow out the other side. The warmed water would flow gently away through the ground—perhaps to be cooled down or otherwise treated elsewhere.
A prototype of this setup is now installed at the chalk quarry. “For nominal pumping rate over the course of about an hour we can shift the temperature of the [bedroom-sized] room down about 10-11 degrees [Celsius],” says Paul. He and his colleagues still need to test how it would perform in large rooms, and, crucially, he’s not sure whether Transport for London (TfL)—the body that operates the Tube—would ever implement it for real.
However, Paul argues the system could be a significant improvement on a similar technology tested by TfL back in 2006. That technology, which is no longer in use, attempted cooling using groundwater that had leaked into‚ and then been pumped out of, Victoria Tube station in central London. Paul suggests that this fluid wouldn’t have been as cool as water directly tapped from nearby aquifers or subterranean rivers. He adds that his system uses special filters to reduce the risk of chalky water causing excessive limescale and blockages.
WIRED asked TfL whether it would consider using the system Paul and his colleagues have come up with. Although TfL declined an interview, Melvin Lim, a spokesman, says the body has had to “carefully prioritize” investments in recent years. He emphasized the introduction of new trains for the Piccadilly Line next year, which will feature air-conditioning: “We stay open to measures that will help manage the impact of increasing temperatures due to climate change.”
TfL, to its credit, has made many efforts over the years to try to deal with the problem of hot tunnels, including attaching cooling panels to tunnel walls. The panels, which circulate water to remove heat from the air, were deployed in a trial in 2022, though they are not currently in use. Paul argues that such a system could be prohibitively expensive.
Hassan Hemida at the University of Birmingham says Paul’s water-cooling techn