Epsom salt originated from a saline spring in Epsom, Surrey, England. These springs emanate when porous chalk mixes with the non-porous London clay. It contains magnesium, sulfur, and oxygen, and has the chemical formula MgSO4.7H2O. This salt has been used for relieving muscle pain, as well as some other medicinal uses like being a laxative, treating nail fungus and athlete's foot. Epsom crystals are quite safe to use and can even be made at home.