Femtosecond lasers are promising tools for micro-machining requiring high surface quality and reliability. One of the many remarkable features in the case of transparent dielectrics is the shallowness of ablated craters, a still unexplained phenomena. We have measured the shape of ablated craters in corundum (alpha-Al2O3) for various pulse duration (from 60 fs to 14 ps). The evolution of ablated craters depth and shape with laser intensity depends on the pulse duration, Below I ps there is a clear threshold above which the depth increases dramatically from 0 to 100 nm, and then slowly increases with laser intensity. For longer pulses. the craters are much more shallow-about 20 nm, and we observe above a second threshold the appearance of a deeper hole in the centre of the first. The fundamental laser-matter interaction mechanisms at work during ablation are discussed. A model aiming to link these microscopic processes and the macroscopic measurements of ablation craters is presented and its results are compared to the experimental data. (C) 2002 Elsevier Science B.V. All rights reserved.