In geomorphology, a minute opening in a rock or soil, through which fluids may pass. Porous rocks allow water to pass through or be stored within them. Pore water pressure is the pressure applied by water in the pores to soil and rock particles, reducing internal friction; when the rock or soil is saturated, pore water pressure can be so great that slope failure results; see Leroueil et al. (2009), Proc. 1st Italian Workshop on Landslides, on slope instability due to pore water pressure increase. However, when the material dries out, only the smallest pores are water-filled, resulting in negative pore-water pressure. The water is held in these pores through strong suction, so that negative pore water pressure can increase the pressure, especially with clays. Negative pore water pressure in soil is referred to as soil moisture tension (or suction).