Hello, A friend of mine told me her dad used to work in the coal mines, like 800m under the ground, and he found that whenever he had to carry something heavy from inside to outside of the coal mine, he always noticed that carrying that same thing underground was easier than carrying it outside (at sea level). I was wondering: is it a plausible explanation that 800m underground there is a higher atmospheric pressure and this somehow leads to an increased oxygen intake in the bloodstream (why exactly? the ratio varies with height?) and as oxygen is used in the muscles for burning, in return of work, this could maybe help him carry the load? Thank you.