  We construct a self-affine sponge in $\mathbb R^3$ whose dynamical dimension,
i.e. the supremum of the Hausdorff dimensions of its invariant measures, is
strictly less than its Hausdorff dimension. This resolves a long-standing open
problem in the dimension theory of dynamical systems, namely whether every
expanding repeller has an ergodic invariant measure of full Hausdorff
dimension. More generally we compute the Hausdorff and dynamical dimensions of
a large class of self-affine sponges, a problem that previous techniques could
only solve in two dimensions. The Hausdorff and dynamical dimensions depend
continuously on the iterated function system defining the sponge, implying that
sponges with a dimension gap represent a nonempty open subset of the parameter
space.
