The design of the reaction mass for the LHPOST departed considerably from conventional design of reaction masses for systems subjected to large impact/dynamic loading, which typically require extremely massive and costly foundations. The approach taken for the LHPOST took advantage of the extremely stiff natural soil conditions at the site. The reaction mass is 19.6 m wide, and 33.1 m long extending to a depth of 5.8 m over most of the area. In a small central area the reaction mass tapers to a depth of 7.9 m to anchor the hold down struts. Given the strong attenuation of the ground motion with distance at the site, the vibrations of the ground will not affect people beyond the immediate vicinity of the table.