Abstract Jefferson Island, an elevation on a dry plain, is one of the Five Islands of Louisiana. The salt dome is under Jefferson Island and Lake Peigneur; the mound was produced by a salt spine, and the lake by subsidence due to solution of a flat table of salt. More than 250 salt exploration, oil exploration, and sulphur exploration and producing wells have been drilled on this dome. Salt was discovered in 1894 in the drilling of a water well, and sulphur was discovered in 1929 in an oil test. The salt is mined from a depth of 800 feet in the salt spine. The sulphur is mined from a deposit between two faults in the cap rock at depths ranging from 600 to approximately 900 feet. More than 2,225,000 long tons of salt have been produced by the Jefferson Island Salt Mining Company, and more than 500,000 long tons of sulphur have been produced by the Jefferson Lake Oil Company, Incorporated, since 1932. Several geophysical surveys have been made on Jefferson Island dome. As a result of the first survey, an electrical one, sulphur was accidentally discovered at a depth of 660 feet in a supposedly deep flank oil test. Later, both torsion-balance and seismograph surveys were made for the purpose of outlining the cap-rock area of the dome. Evidence tends to show that the original cap rock of the dome is of sedimentary origin; however, solution of the salt, as shown by the subsidence of the lake bottom and by drilling, indicates that at least a portion of the cap rock immediately adjacent to the salt is of residual origin. The novel necessity of mining sulphur in the middle of the lake involved several new engineering problems. A floating barge was developed for the drilling of the sulphur wells. The power plant is situated on the shore of the lake. The hot water, steam, air, and sulphur lines are carried to the point of mining operations by means of a trestle. The Frasch process is used in the mining of sulphur. Liquid sulphur from the wells in the lake is pumped to the shore, where it is allowed to cool and solidify in the large vats. It is then blasted down and loaded into cars for shipment.