The Heart Mountain block slide of northwestern Wyoming and southwestern Montana is one of the largest slides known to have occurred in Earth's history. This early Eocene block slide covered an area of over 3400 km2 and moved a minimum of 45 km across open terrain. The initial 2- to 4-km-thick Heart Mountain block slide moved on a slope of about 2°, detaching for half its length on a nondescript bedding plane in the Ordovician Big Horn Dolomite (BHD). Given our current understanding of fundamental mechanics, such a great mass of rock should not have begun sliding on such a gentle slope without some special condition. Here we suggest that a special condition existed during the interval between extensive upper-plate dike injections and the initial movement phase. In our model, the dike injections increased horizontal stresses and heated the surrounding layers. Both the increased stresses and the heat input elevated fluid pressure of water trapped within the BHD. In addition, vertical hydrofracturing was retarded as horizontal stress approached vertical, thus allowing a critical buildup of fluid pressure. Fluid overpressuring is a mechanism that can overcome the mechanical problem of initiating movement on a low-angle surface. Moreover, this mechanism explains the observed fluidized features found along the basal contact of the slide block as well as the observed lack of deformation in the lower plate.