An experimental investigation has been conducted to evaluate the response of rubber blocks under large compressive loadings. Quantitative measurements of the nonlinear behavior of two rubber compounds were tested over compressive strains ranging from 17 to 74 percent. Three distinct phenomena were observed in the deformation behavior of both materials. The first was a dramatic increase in contact area (over 130 percent) between the rubber and the platens, beginning at strains above approximately 33 percent. This increased surface area is used to provide stress corrections in several quantitative models, and appears to physically represent a rolling flow phenomenon, similar to fountain flow in observed in polymer melts. The second and third phenomena are associated with a nonlinear normal stress distribution across the sample's surface. These two phenomena are stress maxima and minima associated with stress singularities predicted by nonlinear elastic models. Deviations in stresses across the specimen surface of up to a factor of four are reported. It is concluded that appropriate constitutive equations describing these phenomena are required before meaningful predictive models can be developed.