The thing that makes the London Underground so hot used to actually make it cold

A hot box.
A hot box.
Image: Reuters/Paul Hackett
We may earn a commission from links on this page.

“Humid, muggy, sultry, sweltering, steamy, oppressive, airless, stifling, suffocating, sweaty, soupy, like a Turkish bath, like a sauna.”

Though that’s a list of synonyms of “sticky” from the Oxford English Dictionary, it also pretty aptly describes what it’s like to travel on the London Underground, the world’s oldest subway system, during the summer. But it wasn’t always this way.

In fact, it was so cool down there around a century ago that its operators actively advertised its chilly passageways, telling commuters that “the Underground’s the only spot for comfort when the days are hot.”

So what happened? It turns out that the very thing that made the Underground refreshingly crisp back then is the same thing that makes the temperature so unbearable now: London’s clay foundations. Raw clay is very good at storing ambient heat, something it then releases slowly when the temperature drops. That meant that most of the heat produced by the trains—which accounts for about 90% of the heat on the Tube—was quickly absorbed by the clay tunnels. But the London Underground has been running for so long, and ridership has been booming in recent years, that the clay can’t take it anymore. It’s literally absorbed all the heat it can.

In the early 1900s, tunnels that had an average temperature of around 14°C (57°F) can now often see air temperatures exceed 30°C (86°F). Earlier this week, during the height of this summer’s heatwave, thermal imaging showed that the Central line, the city’s busiest line, saw temperatures as high as 40°C (104°F). And Transport for London, which runs the Underground, got itself caught in a PR snafu when it non-chalantly revealed in a Twitter Q&A that this line would not get air conditioning for another 12 years.