  Large scale density modes are difficult to measure because they are sensitive
to systematic observational errors in galaxy surveys, but we can study them
indirectly by observing their impact on small scale perturbations. Cosmological
perturbation theory predicts that second-order density inhomogeneities are a
convolution of a short- and a long-wavelength mode. This arises physically
because small scale structures grow at different rates depending on the large
scale environment in which they reside. This induces an off-diagonal term in
the two-point statistics in Fourier space that we use as the basis for a
quadratic estimator for the large scale field. We demonstrate that this
quadratic estimator works well on an N-body simulation of size (2.5 h^{-1}
Gpc)^3. In particular, the quadratic estimator successfully reconstructs the
long-wavelength modes using only small-scale information. This opens up novel
opportunities to study structure on the largest observable scales.
