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Comment by Raidion | Hacker News Reader

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.

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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?

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

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.

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.

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.

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?

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.

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

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.

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.

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.

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

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

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.

Try 390m, by hand:

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

Digging 100m down is difficult and dangerous

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.

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