  The unbiased thermal diffusion of an overdamped Brownian particle in a square
lattice potential is considered in the presence of an externally applied ac
driving. The resulting diffusion matrix exhibits two orthogonal eigenvectors
with eigenvalues $D_1>D_2>0$, indicating anisotropic diffusion along a "fast"
and a "slow principal axis". For sufficiently small temperatures, $D_1$ may
become arbitrarily large and at the same time $D_2$ arbitrarily small. The
principal diffusion axis can be made to point into (almost) any direction by
varying either the driving amplitude or the coupling of the particle to the
potential, without changing any other property of the system or the driving.
