At the present state of knowledge the settlement due to pumping from sand strata located beneath beds of clay can be computed with a reasonable degree of accuracy from the results of soil-tests on undisturbed samples. There is no excuse for not anticipating such settlements, yet it seems to be extremely dif6cult for some engineers to perceive the mechanics of this relatively simple process. During the last 10 years the city of Mexico started to exploit, for water-supply purposes, some of the sand beds located beneath the area covered by the city, with disastrous consequences. Due to the resulting consolidation of the soft clay strata located above the sand, some parts of the city have already subsided more than 10 feet, causing serious damage to buildings and to the sewer-system. The causes of this subsidence are still the topic of lively controversies between the initiators and the adversaries of the theory of consolidation.