If we we dig a hole from north pole to the south pole of the Earth and we throw a ball in this hole where would it stop? Will it come back on the surface or it will stop at the centre?
if u drill such a hole and drop a ball then the result will be that it will harmonically oscillate about the centre of earth with the radius as amplitude
however if ball is directly released at the centre. it will stay still since the net gravitational force is 0.
edit:- as has been pointed by the first commentor , thank you. the answer is for a perfectly uniformly dense spherical ball and that too when the hole u drill is sufficiently small to not alter the mass of earth in any considerable way