Over the course of about 100 years, Venice has lost 23 cm in elevation relative to the average sea level. This decline is attributed to a combination of natural and human factors. A geotechnical study published in 2005 in the Giornale di Geologia Applicata found that 11 cm of this loss is due to rising sea levels, 9 cm is attributed to human activity, and 3 cm is from natural subsidence. The human-caused portion, specifically, is linked to groundwater extraction in the Marghera industrial area between 1930 and 1973. During that period, factories in Marghera pumped up to 500 liters of groundwater per second, significantly lowering the water table and compressing the clay layers beneath the city. This process caused the ground to sink at a rate of up to 17 mm per year in 1969. When pumping ceased in 1973, the ground rebounded slightly by about 2 cm, but 85 percent of the subsidence remains permanent. Before the industrial activity, natural subsidence occurred at a rate of about 1 mm per year, and the most severe period of human-induced sinking occurred between 1950 and 1970. Venice's subsurface contains a high proportion of clay, which made the ground more susceptible to compression when water was removed. The ratio between subsidence and pressure drop in the city was 1/50, compared to 1/100 in Marghera. Between 1930 and 1973, Marghera's pumping caused 14 cm of subsidence in the industrial area, while Venice experienced an average of 10 cm of sinking. Although the city pumped less water than Marghera, the geological composition of the area meant that the impact on Venice was significant. The exact amount of human-induced subsidence is still debated. The 9 cm figure comes from the 2005 study, but earlier simulations from 1974 by Gambolati and colleagues estimated up to 15 cm of subsidence during the same period. Geodetic measurements between 1952 and 1969 recorded about 10 cm of subsidence. Therefore, the most accurate way to describe the human contribution is as a range between 10 cm and 15 cm, depending on the method used. While the cessation of pumping halted further sinking, the 9 cm of subsidence will not be reversed, as most of it is considered irreversible.