Closed Asturian coal shaft in Tuilla, Langreo, retaining a rehabilitated 1950 welded-steel headframe and a deteriorated engine house, both protected by heritage listing.
Pozo El Terrerón, also known as Mosquitera II, was a bituminous coal shaft sunk by the Sociedad Metalúrgica Duro Felguera at Tuilla, in the concejo of Langreo, Asturias. The site was first worked from 1926 within the broader Grupo Mosquitera concession area. Deepening and the construction of the surviving headframe and engine house were carried out in 1950, when Duro Felguera extended the shaft to 480 metres across five working levels.
The headframe, of welded steel construction reaching 41.5 metres in total height, was one of only two shafts sunk by Duro Felguera using the new welding technique at that period. The shaft was connected underground to the adjacent Mosquitera I shaft. On integration into Hunosa in 1967 it became part of the Grupo Siero.
Closure followed in 1988. The headframe was repainted in 2014. The engine house and the washhouse and offices building survive in a deteriorated state.
All three principal structures are listed in the Plan Territorial Especial de Recuperación de los Terrenos de Hunosa en las Cuencas Mineras and in the Inventario Cultural del Principado de Asturias.
The Mosquitera concession area on the boundary of the concejos of Langreo and Siero was first registered in 1875. Early twentieth-century ownership passed through the Unión Hullera y Metalúrgica de Asturias, whose mineral interests in the Candín river valley were absorbed in 1906 by the Sociedad Metalúrgica Duro Felguera. Duro Felguera operated mountain mines in the area until 1926, when it sank the Mosquitera I vertical shaft on the Siero side of the concession. In Tuilla, on the Langreo side, the company began working the site that became Pozo El Terrerón from 1926. This shaft is also known as Mosquitera II, reflecting its role as the second, or auxiliary, shaft of the combined Grupo Mosquitera.
Sinking of the present shaft began in 1942, and production commenced in 1946. In 1950 Duro Felguera installed the current headframe and completed the engine house. The headframe is of welded-steel construction, 41.5 metres in total height, with open-lattice open-box boom struts. This was the second headframe Duro Felguera built using the new welding technique. Its design gave the structure a visual character likened to a compass or tripod, resulting from the use of tubular lattice strut braces with no intermediate bracing between the legs, a technique intended to reduce the weight of the structure. The shaft reached a depth of 480 metres with a diameter of 4.50 metres and five working levels. An intermediate connection between the fourth and fifth levels linked the Terrerón shaft to the Mosquitera I shaft. The engine house built in 1950 is an elegant large-scale building with a pitched roof, decorated externally with pilasters, capitals, and large semi-circular arched openings with decorative impost lines and a prominent pediment, faced in exposed brickwork — a Duro Felguera house style. The washhouse and offices building was constructed between 1932 and 1950, has two floors with a hipped roof, and repeats the theme of semi-circular arched openings and pilasters in a slightly different design, with the exterior combining dressed stone and brick infill panels.
A penal colony for political prisoners was established at the Mosquitera shaft site during the early 1940s, when those serving sentences under the Franco regime were assigned to mine labour.
In 1967 the shaft was integrated into Hunosa and formed part of the Grupo Siero. After its closure in 1988, the fate of the shaft was linked to that of the adjacent Mosquitera I, which closed the following year following its disastrous fire of December 1989.
The headframe survives and was repainted in 2014. The engine house and the combined washhouse and offices building survive in a deteriorated state, with the offices section having partially collapsed in recent years. The three principal structures — the headframe, the engine house, and the washhouse and offices — are included in the Plan Territorial Especial de Recuperación de los Terrenos de Hunosa en las Cuencas Mineras, approved in 2007, and in the Inventario Cultural del Principado de Asturias. Despite this listing, the non-headframe buildings have remained in a state of neglect.