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The London Underground serves as an essential transport system for Londoners, enabling us to traverse the city without wrestling with the congested traffic on the streets overhead. Naturally, the Tube has its disadvantages, but most of us would concur that getting around the capital would be far more challenging without it.

One of the most significant drawbacks of the Underground is how sweltering it becomes, even during the depths of winter. We've grown to accept this as part of daily life, and many of us attribute it to the enormous number of passengers using it daily, plus the trains themselves producing heat as they hurtle through the tunnels.

However, one scientist on social media has revealed that's not completely accurate.

In reality, the Underground wasn't always as scorching as it is today, and there's a scientific explanation for why the tunnels are now frequently intolerably warm.

Professor Hannah Fry, a mathematician, explained to YouTuber Michael Stevens on their The Rest is Science podcast that the London Underground was actually rather chilly when initially constructed, but the clay beneath London's streets can no longer soak up any of the heat generated by the trains.

In a clip posted to Instagram, Hannah explained: "The London Underground is basically a perfect demonstration of the way that the Earth itself can hold onto energy in the form of heat. In the early days when the Underground was built, it was like 10-15 degrees down there. The clay was actually quite cold down there.

"But after 150 years or so of all of these trains moving in and out of tunnels, of people, of that energy being continually pushed in, it's saturated the heat capacity of the clay itself. The clay cannot take on any more heat, effectively.

"All of that heat just continues to be down there. It can be 40 degrees down there in winter! You actually get more people fainting in the Underground during winter than in summer."

Clay possesses a high thermal mass, enabling it to absorb and retain substantial quantities of heat energy. However, the clay within London's underground tunnels has absorbed every bit of heat it can possibly accommodate.

Consequently, this energy now has nowhere to escape and remains trapped within the Underground network itself, forcing passengers to endure the complete intensity of all the heat produced by trains thundering in and out of every station.

Viewers of the Instagram video were astounded by this revelation. Both Londoners and tourists confessed they had been completely unaware of why the Underground was so sweltering, but after viewing the clip, they declared it all made complete sense.

One individual commented: "I remember visiting London in October 2011, it was minus 10 degrees over ground, the Tube I was travelling on was delayed due to someone fainting!".

Another remarked: "Only been to London once and it shocked me how hot and cramped the Underground felt, not just in the trains themselves but the platforms and connecting tunnels."

A third commented: "I was astounded by how hot it gets - and I'm an Australian. No air con. Crazy. I guess that would add to the heat load, though."

Others highlighted that the notion of more people collapsing in winter rather than summer is logical, given most passengers wear heavy coats and scarves to combat the freezing temperatures outdoors.

When they step into the stifling heat of the Underground, they rapidly become overheated.

One person observed: "It doesn't surprise me that more people faint in the winter because they get on the train with their outdoor clothes/jacket and don't remove anything. Standing on a tube with a big, thick jacket on is quite uncomfortable."