Man-made, Underground, partly due to combustion — I would be thinking the nuclear waste site at Yucca Mountain, but Mahaloth said no to radiation. (I also don’t know if radioactive waste actually warms the immediate environment to “sweltering.” My guess is no. Also, without Googling, not sure if Yucca Mountain actually got much/any waste yet.)
Okay, Mahaloth: was the human activity that caused the sweltering temperatures related to the energy industry in a significant way?
No.
If the temperatures were reversed and the recency ignored, then the natural nuclear reactor at Oklo would be a good fit.
Hmmm… is it the Salton Sea?
No.
Is the lowering of water levels involved? Or draining water entirely?
Is it a really deep hole that’s been dug - open-cast mining on a grand scale?
Neither, as far as I know.
No.
Was it an accident?
Is it in the US?
No to both.
Possible reasons for the change:
Transportation, yes.
Is it a tunnel? Is it the Channel Tunnel?
Yes, tunnel.
No to the channel tunnel.
London underground? Has it been getting steadily hotter as its used more?
London Underground? YES
Second question? YES, but why? Not the increase of people or trains.
Does it have to do with how long it has been there? Thermal buildup?
Yep, I think this is solved.
The deepest portions of it are under a cay that was so insulating, the heat is now lingering and unable to escape. When the Underground first opened, it was advertised as an escape from hot weather in the summer. Now, it can be over 30 degrees Celsius.
Oops. Window open too long. Retracting question…
But what about the smell?
I thought I’d read it was very smelly, too, but upon looking, I could not find any reference to this.