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As shown in the figure, some land in the central and western Delta is more than 15 feet below sea level. This situation is caused by land subsidence which is primarily the result of the loss of organic soil (peat). The loss is caused by exposure of peat to oxygen, which converts organic carbon solids to carbon dioxide and aqueous carbon. Subsidence is a major concern in the Delta because it increases the water pressure on levees and, therefore, the probability of levee failure and flooding.