  The impact of dynamical fermions on the vacuum structure of QCD is explored.
Of particular interest is the topological charge correlator, $<q(x) q(0) >$,
where negative values at small $x$ reveal a sign-alternating layered structure
to the topological-charge density of the QCD vacuum. We consider large $28^3
\times 96$ lattices from the MILC collaboration, and develop a new gluonic
definition of the topological charge density, founded on a new over-improved
stout-link smearing algorithm. The algorithm reproduces established results
from the overlap formalism and is designed to preserve instantons. We examine
the extent to which instanton-like objects are found on the lattice. Finally,
we investigate the effects of dynamical sea-quark degrees of freedom on
topology and find that the magnitudes of the negative dip in the $<q(x)q(0)>$
correlator and the positive $<q(0)^2>$ contact term are both increased with the
introduction of dynamical fermion degrees of freedom. This is in accord with
expectations based on charge renormalization and the vanishing of the
topological susceptibility in the chiral limit.
