Extraction of water, oil, and gas from these aquifers has caused land subsidence throughout the Greater Houston region since the early 20th century. Prior to 1942, Houston's municipal water supply was sourced exclusively from groundwater wells. The inception of the petroleum industry at the beginning of the century also led to widespread resource extraction around the city. Surface elevations began to drop with the water table, and by the 1970s, areas around the Houston Ship Channel had subsided up to due to rapid industrialization, prompting the creation of the Harris–Galveston Coastal Subsidence District. By the end of the decade, subsidence had intensified to in some parts of east Houston, and of the region had experienced at least of sinking.