  Diffusive transport is among the most common phenomena in nature [1].
However, as predicted by Anderson [2], diffusion may break down due to
interference. This transition from diffusive transport to localization of waves
should occur for any type of classical or quantum wave in any media as long as
the wavelength becomes comparable to the transport mean free path $\ell^*$ [3].
The signatures of localization and those of absorption, or bound states, can
however be similar, such that an unequivocal proof of the existence of wave
localization in disordered bulk materials is still lacking. Here we present
measurements of time resolved non-classical diffusion of visible light in
strongly scattering samples, which cannot be explained by absorption, sample
geometry or reduction in transport velocity. Deviations from classical
diffusion increase strongly with decreasing $\ell^*$ as expected for a phase
transition. This constitutes an experimental realization of the critical regime
in the approach to Anderson localization.
