[JohnL4]

"Looked the temp gradients up, after 50ft, temp of the earth is 50F, and raises one degree F every 60ft." (So, at 300 feet, 54 degrees; at 500, 57.5 degrees.)

(http://www.welshcoalmines.co.uk/forum/read.php?14,46717,4671...)

"...but other parts of the mine was very cold you made sure you had plenty to ware it depended where you worked"

(https://www.quora.com/Are-coal-mines-cold/answer/Keith-Scott...)

"Underground mines maintain a constant temperature, around 55 degrees Fahrenheit."

(https://www.quora.com/Are-coal-mines-cold/answer/Tom-Inghram)

«The temperature on average is in the 50s, but you still sweat an enormous amount when you start laboring.“ - Alan Bates, working in the coal mines of Letcher County, Kentucky.»

(https://www.quora.com/Whats-it-like-working-in-a-coal-mine/a...)

Yes, a 4000m gold mine is hot. I doubt that's where they're storing their compressed air.

[JohnL4]

And finally: https://startcaving.com/info/caves-are-cold-find-out-how-col...