Closed Palencian coal shaft retaining its distinctive stone-masonry headframe, the only surviving example of its kind in Spain, with a mine interpretation centre nearby.
Pozo Calero, situated at Barruelo de Santullán in the province of Palencia, was the principal coal shaft of the Palencian mining basin and the most important mine in the comarca. Its official name was Pozo Grupo Inferior, but the name of the nearby limestone quarrying area, Calero, became universally used. Sinking was begun in 1910 by the Compañía de los Caminos de Hierro del Norte de España, which had owned the Barruelo hulla workings since the nineteenth century.
The shaft reached 342 metres in 1914, and following delays caused by the First World War reached full-scale production in 1920. Output reached 182,000 tonnes in 1924. After the Spanish Civil War the shaft passed to RENFE, which deprofundised the pozo to 480 metres in 1951.
It closed in 1972 following the electrification of Spanish railways, was reopened in 1993 by Hubasa S.A., and closed definitively in 2002. The shaft is distinguished by its unique masonry headframe of dressed stone, the only surviving stone headframe in Spain. A Centro de Interpretación de la Minería opened in Barruelo in 1999 and displays a detailed scale model of the shaft.
Coal seams at Barruelo de Santullán were first worked during the nineteenth century in mountain mines on the south-east side of the town, in an area known as the grupo superior. By the early twentieth century these reserves were showing signs of depletion, and the Compañía de los Caminos de Hierro del Norte de España, which owned the mines and was the principal consumer of the coal for its steam locomotives, commissioned studies of a deeper potential. Geological surveys carried out between 1908 and 1909 identified the capas of the grupo inferior, located to the north-west of Barruelo, as a new target. Sinking of a new shaft began in 1910 in an open area known as the calero, so named because of limestone quarrying that had been carried out at the site since 1880. The formal name given to the shaft was Pozo Grupo Inferior, but the toponym Calero prevailed. Initial expectations called for a depth of 270 metres and an output of 600 tonnes per day.
In 1912, having reached 90 metres, the work continued, and in 1914 the initial sinking phase concluded at a depth of 342 metres. The outbreak of the First World War that same year delayed the delivery and installation of the winding machinery. Small quantities of coal were extracted during this period using a simple hoist adapted from the sinking equipment, yielding between 60 and 65 tonnes per day in 1917. Full-scale production with the permanent machinery began in 1920, and by 1924 annual output had reached 182,000 tonnes.
In 1922 the Compañía del Norte, seeking to separate railway and mining operations, created a subsidiary, the Sociedad Anónima Minas de Barruelo, to manage the mines autonomously. In 1929 the Sociedad Minas de Barruelo incorporated the Minas de Orbó, unifying ownership of the whole Palencian basin. During the 1920s and 1930s Barruelo's population grew to approximately 9,000 inhabitants, the largest in the province at the time.
The high concentration of firedamp in the seams of the grupo inferior led to severe accidents from the outset of production. Instantaneous outbursts of firedamp — explosions occurring without any ignition source, triggered by minor changes in temperature and pressure in gas that had been trapped for millions of years — were a particular feature of the Calero and were unknown in Spanish mining before they appeared here from around 1920. On 3 May 1930 ten miners died from gas asphyxiation between the 180- and 230-metre levels. On 21 April 1941 an explosion in the ninth seam killed eighteen miners and injured nineteen others. Over the course of its operational life the shaft was responsible for the deaths of close to one hundred miners.
Following the Spanish Civil War the railway companies were nationalised and merged into RENFE, to which the Barruelo shafts were assigned. Because the coal's primary market had been steam traction and RENFE was its only significant customer, the progressive electrification of the Spanish rail network from the 1960s had a severe effect on the economic viability of the mines. In 1951 the pozo was deepened to 480 metres and 22 kilometres of galleries were extended, giving the shaft its definitive dimensions. Between 1964 and 1965 the mines were sold to Hullera Vasco Leonesa, which began closing operations from 1968. The Pozo Calero closed in 1972.
The properties then passed to the Banco de Crédito Industrial, from which Hullas de Barruelo, S.A. (Hubasa), a company that would become part of UMINSA, purchased all the installations of the Barruelo basin in 1980. Hubasa reopened the Pozo Calero at the end of 1993 to extract a coal pillar left by the previous operators as a water protection barrier between the 80- and 130-metre levels. In 1996 the reopened shaft was producing 60,000 tonnes per year and employed 106 workers. Hubasa had by 1998 been integrated into UMINSA, the company owned by the entrepreneur Victorino Alonso. A further deterioration in market conditions led to the definitive closure of the Pozo Calero in 2002.
The shaft's distinctive surface structure is the masonry headframe, the only surviving stone-built headframe in Spain. Constructed from dressed stone, with corner quoins of Quintanilla sandstone, and rising to 14 metres in height, it stands as a unique example of this construction method in the country. Alongside the headframe, the winding engine house, workshops, lamp room, and washhouse survive, built in exposed brick following the architectural traditions of the period. The last mule used in the Palencian mining district was retired in 2002, the year of closure.
A Centro de Interpretación de la Minería was inaugurated in Barruelo in 1999 in the former Escuelas Nacionales building. The centre includes a museum, a visitable mine gallery, and a detailed scale model of the Pozo Calero. The Ayuntamiento de Barruelo de Santullán has actively pursued a Bien de Interés Cultural designation for the shaft, and as of early 2023 had submitted the necessary documentation to the Junta de Castilla y León.