Mineral springs are naturally occurring springs that produces hard water, water that contains dissolved minerals. Salts, sulfur compounds, and gases are among the substances that can be dissolved in the spring water during its passage underground. In this they are unlike sweet springs, which produce soft water with no noticeable dissolved gasses. The dissolved minerals may alter the water's taste. Mineral water obtained from mineral springs, and the precipitated salts such as Epsom salt have long been important commercial products. Some mineral springs may contain significant amounts of harmful dissolved minerals, such as arsenic, and should not be drunk. Sulfur springs smell of rotten eggs due to hydrogen sulfide (H2S), which is hazardous and sometimes deadly. It is a gas, and it usually enters the body when it is breathed in. The quantities ingested in drinking water are much lower and are not considered likely to cause harm, but few studies on long-term, low-level exposure have been done, . The water of mineral springs is sometimes claimed to have therapeutic value. Mineral spas are resorts that have developed around mineral springs, where (often wealthy) patrons would repair to 'take the waters' — meaning that they would drink (see hydrotherapy and water cure) or bathe in (see balneotherapy) the mineral water. Historical mineral springs were often outfitted with elaborate stone-works — including artificial pools, retaining walls, colonnades and roofs — sometimes in the form of fanciful 'Greek temples', gazebos or pagodas. Others were entirely enclosed within spring houses. Types For many centuries, in Europe, North America and elsewhere, commercial proponents of mineral springs classified them according to the chemical composition of the water produced and according to the medicinal benefits supposedly accruing from each: Arsenical springs contained arsenic Lithia springs contained lithium salts. Chalybeate springs contained salts of iron. Alum springs contained alum. Sulfur springs contained hydrogen sulfide gas (see also fumeroles). Salt (saline) springs contained salts of calcium, magnesium or sodium. Alkaline springs contained an alkali. Calcic springs contained lime (calcium hydroxide). Thermal (hot) springs could contain a high concentration of various minerals. Soda springs contained carbon dioxide gas (soda water). Radioactive springs contain traces of radioactive substances such as radium or uranium. Deposits Types of sedimentary rock – usually limestone (calcium carbonate) – are sometimes formed by the evaporation, or rapid precipitation, of minerals from spring water as it emerges, especially at the mouths of hot mineral springs. In