The acceleration due to gravity at a height $h$ above the Earth's surface is the same as that at a depth $d$ below the surface. If both $h$ and $d$ are much smaller than the radius of Earth $R$, then the relation between $h$ and $d$ is:
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Near the surface, gravity decreases twice as fast as you go upwards compared to going downwards. Therefore, to experience the same reduction in gravity, the depth must be twice the height ($d = 2h$).