hmm, gemini and oggyflute posted the exact answer, did you look for the answers on site? cheaters!

I guess they both are correct, and this is what I get..

most people. having never tried to bury a tire, are not aware that one will not stay in the ground. in fact, if you bury a tire five feet below the surface, it will--under normal condition--rise to the top in about ten years.
the reason for this is as follows. the rubber tire, being resilient, is constantly pushing back against the soil around it. and since the pressure above the tire is less then that below it, the tired has more sucess pushing up than it does pushing down. as this pushing proceeds, small particles of soil around the tire are dislodged and fall down through cracks and cervices too small for the tire itself to fit through, a process that is accelerated somewhat by the slight movement of the tire as it expands and contracts with changes in temperature. thus as the tire pushes upward and the soil around it slowly moves down, the tire migrates toward the surface.

now..

ever wonder why most pencils are hexagonal in shape?