  We present a "holographic" reconstruction of bulk spacetime geometry using
correlation functions of a massless field living at the "future boundary" of
the spacetime, namely future null infinity $\mathscr{I}^+$. It is holographic
in the sense that there exists a one-to-one correspondence between correlation
functions of a massless field in four-dimensional spacetime $\mathcal{M}$ and
those of another massless field living in three-dimensional null boundary
$\mathscr{I}^+$. The idea is to first reconstruct the bulk metric $g_{\mu\nu}$
by "inverting" the bulk correlation functions and re-express the latter in
terms of boundary correlators via the correspondence. This effectively allows
asymptotic observers close to $\mathscr{I}^+$ to reconstruct the deep interior
of the spacetime using only correlation functions localized near
$\mathscr{I}^+$.
