The Pozo Sotón mine between Sotrondo and El Entrego in the municipality of San Martin del Rey Aurelio in Asturias province, south-east of Oviedo, offers a rare opportunity to explore coal workings deep underground. Coal was discovered in the region in 1792 by an army engineer Fernando Torres Casado, and the industry was developed from 1845 by the Englishman William Partington, who also founded the first company that supplied gas to Madrid. When his company went into liquidation in 1867 the mines were managed by a French concern, and then, from 1877 by a Spaniard Hermanos Herrero. An ironworks in nearby Langredo, opened in 1859, was one of the principal customers for the coal produced at the mines, and from 1900 the concern was taken over by the ironworking company Duro Felguera. In 1917-22 the shafts were much deepened, and new surface buildings were constructed whose principal features are a pair of 33m high headstocks, together with a power house and offices. The mine made extensive use of compressed air for operating cutting and other equipment. It passed into state control with the Hunosa company in 1967. At its peak the mine employed 1500 men.
Coal production ceased in 2014 but the mine was immediately protected as an historical monument, and is now open to visitors. Two tours are available. One is simply a guided tour of the surface installations, taking in displays about the history of the mine. The other is a formidable undertaking, available only to visitors who are physically fit. It extends over four hours and comprises visits to underground workings at several depths, the deepest at 556 m below the surface.