I do wish posts like this would include their work. There are some highly suspect claims around air pressure. Also, it's almost certainly not okay to use a cons

I do wish posts like this would include their work. There are some highly suspect claims around air pressure. Also, it's almost certainly not okay to use a constant acceleration approximation for g in this case, and I have no idea what equations you'd use for air resistance in those really exotic conditions! Another interesting effect is if the tunnel diameter is large enough, and air is allowed to fill the tunnel, then an arbitrarily large fraction of our atmosphere will now drain into the hole. This effect not only reduces barometric pressure at seal level, but there will be ongoing gas exchange between rock and air. Last but not least, I'd be curious if the magnetic field flux is at all interesting/dangerous to you if you're falling through the molten iron core of a planet. It would also be pretty cool to use that magnetic field as propulsion.