Crack nucleation has been the subject of important contributions in the two last decades. However, it seems that few attention has been granted to the case of saturated porous media. This is the question adressed in the present paper which is devoted to nucleation in traction mode. From a physical point of view, nucleation is a sudden phenomenon, so that the material response is both adiabatic and undrained. In the spirit of the variational approach, the nucleated crack is viewed as the final state of a region of space in which the material undergoes a full damage process. In traction mode, the opening of a saturated crack in undrained condition induces a drop of fluid pressure. In case of low fluid compressibility, the presence of the fluid delays the brittle failure usually associated with nucleation, as long as the fluid pressure remains above the saturation vapour pressure. Nucleation is therefore possible only if a partial vaporization of the fluid takes place.