Objects weigh less the further they go below the earth’s surface.
If you assume that the earth is a perfect sphere with a uniform mass throughout, and you went below the surface of the sphere, only the mass within your radius would contribute to your effective weight.
The radius of the earth is 6,371 kilometers.
If you were able to dig a shaft a kilometer deep, your effective weight would be 6,370 divided by 6,371, or a fraction slightly less than 1 of your effective weight at the surface.
It works like that all the way to the center of the earth.
If the effective radius is zero, your effective weight is zero.
At the center of the earth, all the gravitational forces of the earth would be acting on a person simultaneously, so they would cancel each other out.
