The acceleration due to gravity at a height $h$ (where $h << R_e$) is given by g_h pprox g(1 - \frac{2h}{R_e}). The acceleration due to gravity at a depth $d$ is given by $g_d = g(1 - \frac{d}{R_e})$. Given that $g_h = g_d$: $g(1 - \frac{2h}{R_e}) = g(1 - \frac{d}{R_e})$ $\frac{2h}{R_e} = \frac{d}{R_e} \implies d = 2h$. Given $h = 1$ km, we find $d = 2 \times 1 \text{ km} = 2$ km.