|Prof. Jim Sethna
"If you could drill a tunnel all the way through the earth and dropped a rock in it, what would happen"?
First of all, nobody has ever done the experiment. I think the answer would depend on how they did it!
If you started to dig a tunnel through the Earth and didn't put very powerful, fireproof walls around the tunnel, you'd probably get hot molten lava oozing out.
If you put powerful, fireproof walls on the sides of the tunnel and dropped a rock in, the rock wouldn't fall straight. That's because the Earth rotates around once per day: since it's 24,000 miles around the equator, a rock dropped there is moving at 1000 miles per hour east. The parts of the tunnel close to the center aren't moving as fast, and the center isn't moving at all, so the rock bumps down the east side of the tunnel all the way down, unless your tunnel ran from the North Pole to the South pole.
If you ran the tunnel from the North pole to the South pole and dropped the rock into it, it would heat up from air friction and burn up like a meteor.
If you sucked out all the air in the tunnel, and dropped the rock into it from the North Pole, it would fall faster and faster until it passed the center of the Earth, and then would rise up the other side slower and slower until it just popped up the other side: it would take just about one hour to make the trip.
Answered by Jim Sethna, Physics Professor at Cornell University
Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).