[JohnL4]

I'm guessing not. Earth's heat is way lower down. Think of a cave: always cold. I think what's going on is they're taking the heat out and storing it more efficiently than just dumping it down the ground, where (1) it would leak away, representing energy loss, and (2) as it leaks away, the air would lose pressure. (It would leak away b/c it's harder to insulate an entire mine, basically.) So, they pull it out ahead of time and put it back in when they decompress the air (which I think would add pressure back in due to heating a gas and maybe reduce problems caused by super-chilling other plant hardware? Anyway, seems fitting to add back what you took out if you're shooting for a closed system.)

[Raidion]

This isn't correct. Deep mines are notoriously hot, because you're digging towards magma. Google says temps increase by 3° C for every 100m.

[Melatonic]

Do you have a source for that increase? I was not able to find it. Seems very suspect considering that the earths outer crust is approximately 20 to 30 miles thick. Are you suggesting that if we go approximately halfway through the outer crust, say 15 miles, the temperature is going to be 725 degrees warmer than surface temp?

[wcoenen]

Page 62 of https://archive.ipcc.ch/pdf/supporting-material/proc-renewab...

> The heat is transferred from the interior towards the surface mostly by conduction, and this conductive heat flow makes temperature rise with increasing depth in the crust on average 25-30°C/km

[count]

https://www.nationalgeographic.org/encyclopedia/crust/

Not sure about the rate per-mile, but literally it's that hot, yes, but not even 15 miles. Just down 5-7km under the oceans, for example at the Mohorovičić discontinuity the temp ranges from 392 to 752F.

[Melatonic]

Under the ocean is already much, MUCH closer however to the mantle. Oceanic crust is drastically thinner than the land we walk on.

In the Nat Geo article they quote a mine in South Africa reaching up to 55C (131 F) at the bottom and the mine is 4km deep. At a rate 3 degrees Celsius per 100m it should be 120 degrees Celsius over ambient. Which obviously does not add up.

[m4rtink]

They reached 160C rock when building an industrial railway in Japan:

https://en.m.wikipedia.org/wiki/Kurobe_Seny%C5%8D_Railway

Cooled down to 40C since the 60s.

[emptysongglass]

Naive question but couldn't our forerunners who otherwise succumbed to cold in harsher climates have exploited this fact to dig subterranean villages and towns?

What's the element I'm missing as to why they didn't?

[inglor_cz]

Pre-industrial holes may collapse long before you reach -100 meters. Even -5 meters is a challenge in certain places.

Also, digging (or rather drilling) in bedrock is hard without motorized equipment and good steel.

Also, supplying fresh air down there is a problem.

Also, preventing the mine from flooding is usually a huge problem.

Mining is hard and a lot of people lost their lives doing that. That said, if your only intent is to get a bit warmer, you may basically try a good cave. Caves tend to have temperatures above freezing for the whole year.

[nervousvarun]

And of course the difficulty with caves is hibernating predator(s) (bear is the one I can think of maybe more?)

[inglor_cz]

And lions:

https://en.wikipedia.org/wiki/Panthera_spelaea

IDK if these lions hibernated, but their bones are definitely found in caves all over Europe. Often with human remains or artifacts nearby.

[WiseWeasel]

Difficulty of digging with the tools they had, inability to stabilize the structure to prevent cave-ins, lack of pumps to remove accumulating water, insufficient ventilation and filtration technology/unable to deal with poison gases, inability to light their environment without contributing to the poor air quality, difficulty accessing resources like food and clean water, lack of desire to live in a damp, dusty hole.

[Ekaros]

Also for example Finland can be at least in north considered harsh climate. But these are also areas that suffered of ice age that scrapped most of the softer rocks away leaving only the tougher stuff like granite. Which makes digging very difficult, specially pre-industrial times.

[cortesoft]

That it is really hard to dig down 100m without modern digging equipment.

[PixelOfDeath]

Ever tried to dig down a few 100m in your garden?

[mikewave]

More like, ever tried to dig down more than a foot or two? Unless you have really soft ground, even a good steel spade and shovel aren't any guarantee of success. Powered augers are often needed to make holes for fence posts, etc.

[m4rtink]

Try 390m, by hand:

https://en.m.wikipedia.org/wiki/Woodingdean_Water_Well

[sulZ]

Digging 100m down is difficult and dangerous

[andrewflnr]

Have you been in a cave? They're chilly, usually around 50F, year round. IIRC those "deep mines" are a LOT deeper than any compressed air storage system is likely to be.

[benreesman]

This comment is an example of why I’ve become skeptical of the downvote button. It sounds like people read it, knew better, and politely pointed out that it was incorrect.

Everyone who might have shared the same misapprehension learned something and the parent wasn’t being an asshole.

Doesn’t that add to the discussion?

[nkurz]

Yes, when considered along with the corrective responses, it adds to the discussion. This is why it shouldn't be flagged and removed. On the other hand, it's factually wrong and would be misleading if people were to trust the claims uncritically, which I think justifies a downvote. Keeping it visible but lower on the page and showing it in gray seems like a pretty good compromise. Can you suggest a better way of giving more prominence to correct information? Or do you think that's not the right priority for the site?

[ndm000]

Side question - do you need to reach a certain level of membership with HN to see the downvote button? I may be missing something, but seems that's only allowed for certain individuals.

[nkurz]

Yes, there's a minimum number of points required to downvote. I think it's currently 500. Here's an unofficial FAQ that give more information about the thresholds: https://github.com/minimaxir/hacker-news-undocumented/blob/m...

[nautilius]

Didn't downvote, but I am not a big fan of simply making stuff up on the fly as response. I've met my share of people who rather go this route than put in a 5sec Google search or simply admit they don't know something, and I try to avoid them.

[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...

[OneLeggedCat]

> Earth's heat is way lower down

Very incorrect.