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I didn't read all the previous reply but:
I had the same question and I asked my Physics teacher about it.
If you make this hole and there's no friction, air resistance or any other things that would make you lose energy. All your potential energy from the point where you jump would equal your cinetic energy when you fall. This mean that if you jump at sea level on the surface of the earth, you would fall through that hole until you reach the center and right there your speed would decrease. You would logically come out at the exact same height from where you jump but at the opposite side of the earth.
But if there's air resistance or anything, you would oscilliate from one side of the earth to the other until you arrive at the exact center of the air where you would ''fly''. You would be weightless since there would be no forces applied on your body.
Hope it's clear cause it's kinda hard to explain.