  If $\Gamma$ is the nullity space of the curvature tensor of a Riemannian
manifold $M^n$, it is well known that if its dimension is constant and if $M^n$
is complete then the distribution $\Gamma$ is completely integrable with flat
leaves. The case of $\dim \Gamma = n-2$ are the so called conullity two
manifolds which naturally arise in various geometric contexts. The obstruction
to the metric splitting isometrically is a 2x2 matrix which is either nilpotent
or invertible. We study the case where it is nilpotent, or equivalently where
the scalar curvature is constant along the leaves of $\Gamma$. When $M^n$ is
locally irreducible we show that $M^n$ admits a Lipschitz foliation $F$ by
totally geodesic flat hyperplanes and determine the metric on a naturally
defined open dense subset in terms of $n-1$ functions, uniquely determined up
to isometry. We can also find examples of smooth complete metrics of conullity
two where the foliation $F$ is smooth only on the complement of a Cantor set.
We furthermore show that the fundamental group is either trivial or infinite
cyclic.
