I enjoyed Bill Larson's pointing out that the rapidly increasing density
of air easily becomes the biggest impediment to the body-oscillating-in-a-
hole-drilled-through-the-Earth problem in the real world, a twist I had
never before heard. Indeed, with a scale height of 7 or 8 km, atmospheric
density would increase to that of liquid water at the bottom of a hole
just 50 km deep. Want the density of mercury? Just continue boring
another 20 km down. At some point before this, of course, the phase and
the equation of state will be significantly altered. I love the image of
looking down a long shaft into a pool of liquid air.