The accuracy required in the positioning of magnet units of the large CERN Proton Synchrotron imposed preliminary studies of the stability of the superficial ground and of the deep layers before designing the proper foundations. Repeated precise surveys, above and under ground, carried out with a O·1 mm. accuracy, brought to light periodic expansions of the subground (10−6 in relative size), connected with periodic tiltings (10−6 radians), both phenomena being indirect effects of Atlantic Ocean tides. These movements made it necessary to contrive elastic foundations for the magnets. The study of their stability, together with optical refraction considerations, led to the conception of.the method to be used to position the units. A final accuracy of ±0·125 mm. r.m.s. was achieved around the 100 metres radius circle. As a conclusion, possible methods of surveys for future larger synchrotrons are considered.