Hi SpoonR,

I'm not sure how the spire actually couples to the ground, but I could speculate on a couple things with you without going back through the book.

I was under the impression that it is essentially a soup can on the ground, though I speculate that either the habbles don't really reach the ground, or the ground-level habbles are uninhabited.  At some point the book mentions how much of the spire is actually inhabited, but I don't recall.

As for the atmosphere... The book at some point describes [Ee]arth.  I can't remember if it's used as a proper noun or not, though I chose to view the planet at that point as very Earth-like.  Given that choice, I beg to differ about the oxygen deprivation experienced at 2 miles altitude.  I believe that is an altitude that can be readily adapted to.  If not by any Joe Shmoe, it could at least be adapted to through generations.  If you look into Everest climbs, I think you'll find that the local people can ascend to significantly greater altitude than the visiting climbers can without enriching the air they breathe with oxygen.  The sheer statistics involved in bonding oxygen molecules with hemoglobin with low O2 partial pressure are inescapable, but I believe the ways our bodies react to different oxygen concentrations in the blood can vary.  My guess is that Dina knows way more about this than I do =).

As you have touched on with the comment about more oxygen vs more pressure, it is the oxygen partial pressure that's important (i.e., the pressure due only to oxygen molecules).  Atmospheric pressure vs altitude quite fairly precisely follows an exponential decay curve.  However, partial pressures are more complex (e.g., ozone layer).  I wonder if perhaps the high water pressure (mist) in the lower atmosphere affects the oxygen pressure in a way favorable for the altitudes you are talking about?

It's fun to speculate about =).