The presence of a salt sheet in the subsurface provides a permeability barrier to the flow of fluids, which can lead to the development of an overpressure condition and slower rate of compaction for formations underlying the salt sheet. These effects are demonstrated through (a) numerical simulations and (b) observations from four wells in the Gulf of Mexico which penetrated salt sheets and tested the underlying formations. The numerical models simulate the time-dependent fluid flow in a sedimentary basin containing a salt sheet; these models predict a significant increase in excess pore pressure in subsalt formations relative to the regional trend and also relative to the formations overlying the salt. The well data confirm the model results. Based on analysis of sonic log, density log, and mud weight data, the subsalt formations in three of the four wells are determined to be extremely overpressured while the fourth well is also highly overpressured, but not to the same extent. This work shows that pore fluid pressure constitutes a significant risk component in assessing a subsalt prospect and needs to be analyzed carefully before drilling.