Pore pressures below hydrostatic (underpressures) have been measured at the Wellenberg site in Switzerland, that Nagra (Swiss National Cooperative for the Disposal of Radioactive Waste) has selected as a potential repository for short-lived radioactive waste. The rock consists of overconsolidated Cretaceous marls and Tertiary shales, characterized by low to very low hydraulic conductivity. The maximum measured underpressures are between 4.8 and 7.1 MPa below hydrostatic and occur at depths of ~800 m below ground. Results from this investigation show that mechanical unloading is capable of producing the observed underpressures. Unloading can be produced in conjunction with denudation (uniform erosion), differential erosion, and glaciation. The effect of these processes, of relevant material parameters, and of boundary conditions are investigated in two phases. In the first phase, simple one-dimensional and two-dimensional column models are used under uniform erosion (denudation) conditions. These investigations show that within the range of the investigated parameters and physical processes, the hydraulic conductivity, the stiffness, and the drainage boundary conditions are more important than erosion rate, horizontal stresses, and nonlinear material behavior. However, although underpressures are produced with parameters and conditions corresponding reasonably well to observations, uniform rates of denudation alone cannot account for the actually encountered magnitude of underpressure. The second phase in which differential erosion and, particularly, the effect of glaciation are considered produces very realistic results. This is established by examining a number of combinations of denudation, differential erosion, and glacial loading and unloading. ¿ 2001 American Geophysical Union