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Context is an alternate reality where at some point in time, one large tunnel appears on Earth.

Both ends of this tunnel are 100m above sea level, respectively located at most eastern equatorial point of Africa, and most western equatorial point of Africa.

The tunnel does not follow Earth's curvature. It is one perfectly straight tube, therfore it slowly plunges into Earth's crust and reaches its deepest point at its midpoint, 271km below surface, in Earth's red hot mantle. It spawns already filled with atmosphere, so there is no devastating hammer effect of two fronts of air rushing in the vacuum of this tube.

It is an amazingly wide 1km inner diameter, and it has 0.5km thick walls made of latest UnobtaniumZeroLambda material.

UnobtaniumZeroLambda properties are:

  • able to withstand temperature and shear stress of mantle's molten rock convective displacement.

  • locally able to vary its density in real time, to soothe the archimedean attempts at deforming the crust and creating earthquakes at both ends.

  • perfect insulating material, allowing strictly 0 watt per meter Kelvin to pass through. (which means Earth's mantle has no influence at all on atmospheric temperature inside the tunnel)

enter image description here (source googleearth)

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Question is mainly related to science based observations regarding the atmosphere inside the tunnel :

What would be average atmospheric parameters inside the tunnel (temperature, pressure, state) assuming standard and equivalent atmosphreric pressure and temperature at both ends?

What could be conditions inside the tunnel if there is huge difference in temperature and pressure at both ends? (Earth's terminator at tunnel's midpoint + anticyclonic condition at one end, sun aligned along tunnel's axis at equinoxes)

By extension, would it be possible and what would be required to safely travel through it? (by foot, car, train, airliner)

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  • $\begingroup$ Is the tunnel completly open, unobstructed to the atmosphere at each end? Assuming that the climate conditions are unchanged from normal, I can imagine there would be an air pressure and an air temperature dfference between the two end points, owing to normal atmospheric highs and lows, sunshine, air currents, and such. Given a one kilometer diameter tunnel, I can imagine a tremendous hurricane-force wind blowing through the tunnel. There would also be a pronounced Coriolis force. The tunnel would spin with the Earth's rotation, shooting air out one end. For sure, the air would not be static. $\endgroup$ – Justin Thyme the Second Sep 30 '20 at 0:34
  • $\begingroup$ What is the friction coefficient of the walls? How smooth are they? Would there be a laminar air flow through the tunnel? Could any transportation vehicle make friction contact with the walls, or would it be one very long slip-and-slide? $\endgroup$ – Justin Thyme the Second Sep 30 '20 at 0:37
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    $\begingroup$ Please note that one end of your tunnel is at the birthplace for most hurricanes that form in the Atlantic. $\endgroup$ – Justin Thyme the Second Sep 30 '20 at 0:42
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    $\begingroup$ @JustinThymetheSecond as you said wind always blows from high to low pressure. Since highest pressure will always be in the middle point of the tunnel, it is unlikely that there will be air circulation from one end to the other. $\endgroup$ – qq jkztd Sep 30 '20 at 7:10
  • $\begingroup$ @qqjkztd Not that simple. Look up catabatic winds -- which always flow from a highland to a lowland, by definition. Air pressure is always lower at the source than the sink, but the wind still blows, driven by relative density (cooling, sinking air in the highland flowing through a gap in boundary mountains). Further, though the tunnel has a huge pressure at the midpoint (see my answer below), a pressure differential between the ends will still drive circulation -- it'll just have a really low velocity at the midpoint (at least until water blocks the tunnel) due to the compression there. $\endgroup$ – Zeiss Ikon Sep 30 '20 at 11:58
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Tunnels of this conformation (geometrically straight and therefore dipping into and through crust and mantle) have been semi-seriously proposed as transportation systems. They have the advantage that (according to math I can't claim to understand) gravitationally driven, unpowered travel through such a tunnel (evacuated, with frictionless rails) would take a constant time for any transit, regardless of end points.

For a tunnel open at the ends, the most salient issue would be atmospheric pressure. You calculation shows 271 km depth at the midpoint -- that's more than four times the depth of the troposphere on Venus, indicating that you'll have a pressure near 400 atm at the midpoint (a bit less, perhaps, because Earth's gas mixture is less dense than the cardon dioxide that makes up most of the Venerian atmosphere). That's enough pressure to make the tunnel impassible to current travel technology -- we don't even have submarines that are self-powered that can stand the equivalent of 40 km depth of sea water. And then there's temperature -- not due to heat flux through the (perfectly insulative) tunnel wall, but due to simple pressure/temperature relationships. The same lapse rate that makes it cooler in the mountains than at the beach will make it hellishly hot by the time you're a few tens of kilometers into the tunnel (and a kilometer or so below sea level).

Further, since the air in the tunnel will interchange with the air outside, and the size of the tunnel (far larger than airship hangars, the overall largest enclosed structures), there will be weather inside the tunnel. Lack of solar heating suggests that much of this weather will involve deposition of water introduced with inflowing air (rain, fog, condensation on the walls). Additionally, air can hold less water vapor at higher pressure. So there'll be water inside the tunnel, and we literally have no reliable idea what happens to ambient temperature water at 400 atm. Too hot to make pressure ice, I think, quite possibly supercritical (or near critical -- read Close to Critical by Hal Clement for how that would act).

A kilometer of rain? Impossible! But it won't take a kilometer of rainfall to close the tunnel; it will take a small fraction of that amount, because the midpoint is gravitationally "down" from all other points in the tunnel, so all the rainfall within the tunnel (as well as "natural" rain that fall into either opening) will run down to the center. In a way, it won't matter; no human will ever see the midpoint of the tunnel.

To be perfectly clear: this tunnel is not a horizontal tube. Horizontal would follow the Earth's reference datum (sea level, which is very nearly spherical; its oblateness due to Earth's rotation doesn't matter here); you can clearly see from the diagrams in the question that, relative to the surface at either end (presumed level ground) the tunnel slopes by several degrees -- and if it slopes down toward the midpoint at both ends, the midpoint must be the bottom. Don't let the geometric straightness of the tunnel fool you; the midpoint is genuinely 271 km below sea level as it would exist directly above the midpoint.

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