I've seen a lot of explanations about digging a hole through the earth.....but this on from Neil deGrasse Tyson / NOVA scienceNOW NOVA Short Journey Through the Center of the Earth is the best!!
(mute sound from playlist popout below before viewing video)
For lesson plans related to this subject, view my 2/21/09 blog post:
How to dig a hole to the center of the earth