Functioning mine museum at Arnao featuring the oldest vertical shaft in Asturian coal mining, a Bien de Interés Cultural headframe, and accessible submarine galleries.
The Pozo Maestro de la Mina de Arnao is the principal shaft of the Mina de Arnao in the concejo of Castrillón, Asturias, on the coast of the Cantábrian Sea. It is distinguished by three exceptional characteristics: it is considered the oldest documented coal mine in the Iberian Peninsula, the first vertical shaft sunk in Asturian coal mining, and the only submarine coal mine in Europe. The coal seam runs beneath the sea floor, and the mine's galleries extend under the ocean.
The Real Compañía Asturiana de Minas (RCAM) opened the colliery with industrial methods in 1833, sinking the first vertical shaft of the Asturian coalfield. The current wooden headframe, clad in zinc, dates in its present structure from around 1902 following a rebuilding by architect Tomás Acha Zulaica. The mine closed in 1915 due to progressive marine water infiltration.
The headframe and associated buildings were declared Bien de Interés Cultural with the category of Monumento in 2011. Restoration works begun under the Ayuntamiento de Castrillón allowed the Museo de la Mina de Arnao to open in 2013, offering guided visits to the headframe, the original shaft cage, and accessible sections of the underground galleries.
The first documented reference to coal at Arnao dates from 1591, when the friar Agustín Montero wrote to King Felipe II to report the discovery of a black mineral in the coastal cliffs and to request a licence for its exploitation. The king granted the concession, and a brief extraction was carried out. Two galleons loaded with coal were dispatched from Avilés to Lisbon before the enterprise ceased.
This early activity constitutes the first documented exploitation of mineral coal in the Iberian Peninsula. No further significant activity took place at Arnao for more than two centuries. The modern industrial phase began in 1833, when the Real Compañía Asturiana de Minas de Carbón was founded, with Belgian and Spanish capital, by Ferrer, Riera, and Lesoinne.
The Belgian industrialists Nicolas Maximilien Lesoinne and his son Adolphe received from Queen María Cristina de Borbón-Dos Sicilias a concession for exploitation over 25 years and the right to use the royal coat of arms on the company's buildings and products. The RCAM began operations at Arnao in 1833, with 11 workers in that first year, rising rapidly to 38 in 1834 and 130 by 1835, including for the first time three women workers — Victoria, Josefa, and Teresa — the first women employed in mining in Spain. The company sank the first vertical shaft in the Asturian coalfield, an event that is dated to 1833 in some accounts and associated with a headframe first erected in 1856 in others; the shaft reached an initial depth of 80 metres, with a submarine gallery of approximately 250 metres.
Over time the mine was deepened to 250 metres, with kilometre-long galleries extending beneath the seabed. The RCAM introduced numerous technical innovations at Arnao that were subsequently adopted across the Asturian coalfield: blasting with gunpowder, systematic coal cutting, mineral railways (the first animal-powered railway in Spain, built at Arnao in 1836, connected the mine to a small embarcadero), compressed air ventilation, and later steam haulage. In 1835 the RCAM also obtained concessions over the coal seams of Santa María del Mar, expanding the scope of its submarine workings.
In 1851, the Belgian Jules Hauzeur presented to the company a study on the feasibility of a zinc smelter at Arnao. A zinc factory was constructed in 1853 by a subsidiary, Asturiana de Zinc, diversifying the industrial complex and making Arnao the site of the first zinc smelting works in Spain. In 1858 Queen Isabel II, accompanied by her husband Francisco de Asís de Borbón, visited Arnao and descended in the cage to inspect the underground galleries.
In 1880 the RCAM purchased the locomotive Eleonore, a narrow-gauge steam locomotive that is the oldest surviving narrow-gauge locomotive in Spain and is now preserved in the Museo del Ferrocarril y la Industria in Gijón. From 1899 the mine's installations were at full operation, incorporating comprehensive underground maintenance systems. In 1899, or more precisely from 1902 according to the declaration documentation, the existing headframe was rebuilt; its current form, with a wooden frame clad in zinc scale sheeting with a lantern temple at the apex and zinc cresting and pinnacles, is associated with a reconstruction undertaken after Tomás Acha Zulaica joined the RCAM in December 1902.
From around 1903 difficulties increased owing to progressive infiltration of seawater into the submarine galleries. These infiltrations worsened over the following decade, and in 1915 the decision was taken to close the coal mine. The zinc factory continued to operate independently.
The mine buildings and headframe stood abandoned for most of the twentieth century. By the time of the first interventions in the twenty-first century, the galleries had been without oxygen and had accumulated significant water. The conjunto histórico industrial de Arnao was included in the Inventario del Patrimonio Cultural de Asturias by resolution of 12 July 2007.
An expediente for the declaration of the castillete as Bien de Interés Cultural was opened in September 2009. In 2011, under the Principado de Asturias Decreto 21/2011 of 9 March, the castillete del enclave minero de Arnao was formally declared Bien de Interés Cultural with the category of Monumento. The rehabilitation project was financed principally by the Ayuntamiento de Castrillón with 70% of the costs drawn from FEDER funds of the European Union (3.5 million euros out of a total of 5 million euros).
Restoration works were undertaken by SADIM, an engineering company of the HUNOSA group, and involved consolidating the shaft headframe, stabilising and opening over 100 metres of gallery, and creating an exhibition space in the former mine buildings. In July 2013 the Museo de la Mina de Arnao opened to the public. Visitors may descend via a glass-sided lift to access the underground galleries, inspect the original shaft cage preserved within the headframe, and visit the interpretation centre installed in the surface buildings.
The zinc factory continued to operate under successive company names and is now part of the XSASTRA group.