Earth tide induced by the Moon and the Sun makes the volume of pores or the apertures of fractures of the bed rock vary, which in turn makes the ground water move. The flow equation of ground water, complete with the tide effect, and its solution, shows that the diffusive propagation of pore pressure is decisive for the response of the perturbation of gravity. Moreover the solution shows that diffusive flow dominates in the Fennoscandian Precambrian rock. The analysis shows that a constitutive coefficient relating the varying perturbation gravity and porosity of bed rock can be determined by field measurements of the level of water in boreholes. Water in a cavity in the bed rock is exposed to an oscillatory interaction with the surrounding pore water. If the cavity is a repository for nuclear waste, this interaction deserves attention. The analysis shows that the magnitude of the oscillatory motion of water between a repository for nuclear waste and its. surrounding groundwater is much less than the vertical motion of the phreatic surface.