Question

Why weight decreases when we go beneath the surface of earth?

Answer 1

As you go down below the Earth's surface, in a mine shaft for example, the force of gravity lessens. Weight and gravitational pull continue to decrease as you get closer to the centre of the Earth.

If you're right in the centre, if that were possible, and you've got the Earth surrounding you, then you're being pulled equally in all directions and the net effect is that they cancel out. There's no gravitational pull and you'd be weightless.

But your mass never changes.