Salt became an engine of commerce and wealth for Zigong. There were cheaper methods for salt production, as to start a borehole well cost 3,000 pieces of silver, but transportation cost into the Sichuan valley negated the cheaper production methods (as they usually involved sea water). Zigong is located in the central Chinese Sichuan province, which was a large basin, surrounded by mountains on three sides and therefore somewhat isolated. Before refrigeration was available salt was a key chemical used in preserving vegetables, meat, and fish. Because of the factors mentioned above (war, wells drying up, and a rebellion) Zigong became a significant supplier of salt to the Sichuan province up until the 1930s. In 1892, miners discovered a vein of rocksalt that feeds Zigongs aquifers. In 1946, a well was drilled with a rotary drillbit. Ancient percussion drilling methods became obsolete and most of the drilling rigs were torn down due to decay. Today, only one traditional salt well still exists, the Shenhai Well, as a tribute to the workers and industry. The Shenhai Well (Bore depth: 1,001.4 meters/4,400 feet) still operates using the older manual methods of pumping and boiling. The salt is used for pickling or canning - and makes an important ingredient to Sichuan style pickles (which are a local favorite). In the modern day, Zigong is a primary source for Natural Gas, Coal, and Inorganic Chemical Production in addition to salt.