Okay, so I just got out of the shower. While in the shower, I had this strange thought; if it was actually possible to tunnel from here to China through the earth, would you ever get there? After you got to a certain point, the opposing gravity would push you back towards where you came from (in my case, Canada). And if there was already a tunnel from Canada to China and you jumped into it, you'd probably get stuck about halfway. Gravity from both sides of the earth would each be pushing you "down". There are only two possible outcomes in this situation: 1) You hang there in midair until you are somehow rescued. Or 2) You implode. So much for digging that hole to China.