On Mon, Oct 17, 2016 at 02:27:04AM -0400, Richard Aiken wrote:
> If we bump the sea level pressure up to 0.34 atmospheres (just below the
> lower limit for unassisted breathing) and leave the temp at -60 C, then we
> need a depth of -12,500 meters to reach 0.90 atmospheres.

8 km depth should suffice for that ratio, at 0 C.  In reality the
temperature changes with height, making the calculation more
difficult, but that's a decent approximation.

It also matches up well with the actual measured pressures at the top
of 8 km mountains.

- Tim