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Extracted sulphur. Sulphur which has been extracted from crude natural gas known as sour gas. Sour gas is rich in hydrogen sulphide (formula H2.S). Before sour gas is used, the hydrogen sulphide impurity is broken down into inert sulphur. The sulphur can then be marketed as a by-product. Photographed in Alberta, Canada. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAVID NUNUK)