  We embed the component fields of eleven-dimensional supergravity into a
superspace of the form $X\times Y$ where $X$ is the standard 4D, $N=1$
superspace and $Y$ is a smooth 7-manifold. The eleven-dimensional 3-form gives
rise to a tensor hierarchy of superfields gauged by the diffeomorphisms of $Y$.
It contains a natural candidate for a $G_2$ structure on $Y$, and being a
complex of superforms, defines a superspace Chern-Simons invariant. Adding to
this a natural generalization of the Riemannian volume on $X\times Y$ and
freezing the (superspin-$\frac32$ and 1) supergravity fields on $X$, we obtain
an approximation to the eleven-dimensional supergravity action that suffices to
compute the scalar potential. In this approximation the action is the sum of
the superspace Chern-Simons term and a superspace generalization of the Hitchin
functional for $Y$ as a $G_2$-structure manifold. Integrating out auxiliary
fields, we obtain the conditions for unbroken supersymmetry and the scalar
potential. The latter reproduces the Einstein-Hilbert term on $Y$ in a form due
to Bryant.
