The Senda del Oso, or Bear Trail, is one of the most well-known outdoor routes in Asturias and one of the most interesting former railway trails in northern Spain. It runs through the valleys of the central Asturian mountains, following the course of the rivers Trubia, Teverga and Quirós, crossing forests, tunnels, bridges and narrow limestone gorges. Its name is closely connected with the Cantabrian brown bear and, in particular, with the famous rescued bears that have lived in the enclosure at Proaza for many years.
The route follows the trackbed of an old mining railway constructed during the early 20th century to transport minerals, particularly coal, from the mining areas of the interior towards the industrial centres of Asturias. The railway connected the valleys of Teverga and Quirós with the wider railway network around Trubia, where the minerals could be transferred for further transport. Several sections were built through difficult mountainous terrain, requiring tunnels, bridges and substantial engineering work to cross the narrow valleys. Following the decline of mining and the eventual closure of the railway, the old track was progressively converted into a public greenway for walkers and cyclists. The first section, between Tuñón and Proaza, opened in May 1995, followed by the extension towards Entrago in 1996 and the connection with Valdemurio in 1999.
Today, the Senda del Oso is officially designated as both a pedestrian and cycling route, so cycling along it is perfectly permitted. The route passes through the municipalities of Santo Adriano, Proaza, Quirós and Teverga, with several access points and different branches allowing visitors to choose how much of the trail they wish to cover. The main section between Tuñón and Entrago is approximately 20–22 kilometres long, although the total distance depends on the branches and extensions included. One of the most frequently used starting points for cycling is Entrago, in Teverga, from where the route continues downhill towards Buyera and Tuñón.
The old railway alignment makes the route particularly suitable for recreational cycling because it has a relatively gentle gradient and does not involve the steep climbs normally associated with mountain routes. The surface is mostly paved and the trail is separated from ordinary road traffic, although cyclists must still share it with pedestrians and should reduce their speed when approaching walkers, families and other cyclists. Along the way there are numerous tunnels excavated directly into the limestone, bridges over rivers and streams, wooded sections and several recreational areas where it is possible to stop for a break.
One of the most recognisable sections is the Desfiladero de Peñas Juntas, a narrow gorge where the limestone walls rise close to the former railway line. Other parts of the route follow the River Trubia through green valleys, passing small villages and stretches of woodland. The tunnels are among the most characteristic features of the trail, as they preserve the original railway infrastructure and provide a direct reminder of the difficult conditions in which the mining railway was constructed. Some are relatively long and can be dark, so bicycle lights are useful, even though the route has several illuminated sections.
Cycling is probably the most convenient way to cover the longer sections of the Senda del Oso, particularly when starting from Entrago. Several local companies offer bicycle rental, and one of the established operators is TeverAstur, based at Entrago, directly beside the beginning of the route. The company provides different types and sizes of bicycles, including mountain and trekking models, and supplies a map with information about the route, distances and main points of interest. Helmets and assistance in case of mechanical problems are also included in the rental service.
The most popular option offered by TeverAstur is the downhill section between Entrago and the recreational area of Buyera, covering approximately 16 kilometres and usually taking between two and three hours at a relaxed pace. It is a relatively easy ride because the old railway follows a gradual downhill profile. There are no guided groups to follow, so riders can proceed independently and stop whenever they wish to explore the surroundings, take photographs or have something to eat.
One particularly useful feature of this service is the return transfer. After completing the ride at Buyera, the company collects both the cyclists and their rented bicycles and transports them back uphill to Entrago, where they started. The return service is included in the bicycle rental, so there is no need to cycle back up the same route. TeverAstur operates return journeys from the lower section from approximately 13:00 onwards, with several available departure times. Another longer option extends the ride towards the Valdemurio reservoir before continuing to Buyera, covering approximately 26 kilometres in total. This version includes an uphill section and requires more time and physical effort.
One of the main reasons why the Senda del Oso is so famous is the presence of the Cercado Osero de Proaza, a large semi-natural enclosure for rescued Cantabrian brown bears located directly beside the route, between the localities of Buyera and Tuñón. It is possible to reach the viewing area either on foot or by bicycle, and the enclosure has become one of the most recognisable attractions along the greenway.
The enclosure was inaugurated in 1996 and was originally created to provide a permanent home for two female Cantabrian brown bears, Paca and Tola. Both animals had been rescued as very young cubs in 1989, after a poacher killed their mother in the mountains of Tineo. Because they had become accustomed to human presence and could no longer be safely returned to the wild, they were transferred to Proaza and spent the rest of their lives in the protected enclosure.
The two bears became particularly famous throughout Asturias and were closely associated with the development of the Senda del Oso as a tourist destination. Tola died in January 2018, at the age of 28, while Paca died on 10 April 2025, at approximately 36 years old. Their story played an important role in raising awareness of the problems facing the Cantabrian brown bear and the importance of protecting the species.
Another female bear, Molina, arrived in December 2013 after being rescued as a cub in the Picos de Europa. She was subsequently introduced to the enclosure with Paca and Tola and adapted to life in the semi-natural environment. Following Paca’s death in 2025, Molina became the only resident bear in the best-known viewing enclosure at Proaza.
The enclosure covers more than four hectares of woodland on Monte Fernanchín. It is not a conventional zoo with small cages or a restricted exhibition space. Instead, it provides the bear with a much larger area containing vegetation, rocky ground, trees, natural shelters and places where she can move away from the visitor viewing points. The intention is to allow the animal to maintain as many natural behaviours as possible while remaining in a protected environment. A second enclosure was also constructed in 2008 on the opposite side of the Senda del Oso, with facilities designed partly for the temporary rehabilitation of wild bears before their possible return to the natural environment.
Visitors cannot enter either enclosure or approach the animals directly. Observation takes place from the public viewing area beside the route, where it is sometimes possible to see Molina moving through the vegetation or resting in the open areas. However, sightings are not guaranteed because the enclosure is extensive and the bear can move into wooded sections that are not visible from the viewpoint. Quiet behaviour is important, as shouting and disturbing the animal can cause her to retreat further into the woodland. Feeding the bear is strictly prohibited.
The viewing point is free of charge and accessible throughout the year. The best opportunities to see Molina are generally early in the morning or later in the afternoon, when bears are often more active. During the warmer part of the day they may spend long periods resting in shaded areas, while in winter their activity can decrease considerably and they may remain inactive for extended periods.
The area around Proaza is also important for the conservation of the wild Cantabrian brown bear. Unlike the animals in the enclosure, wild bears live across the forests and mountain ranges of the Cantabrian Mountains, moving between wooded valleys and high-altitude areas in search of food. The species was historically affected by hunting, habitat fragmentation and human persecution, but conservation measures have contributed to its recovery. The Fundación Oso de Asturias, established in 1992, plays an important role in the protection of the species, environmental education, research and the care of rescued bears.
In Proaza itself, the Casa del Oso is the main interpretation centre dedicated to the Cantabrian brown bear. The building houses exhibitions explaining the biology, distribution and conservation of the species, together with historical material about the rescued bears and the work carried out by the foundation. It provides additional information about the difference between bears living in semi-natural enclosures and those surviving independently in the Cantabrian mountains.
The combination of the old railway route, the mountain landscape and the bear enclosure makes the Senda del Oso a particularly interesting destination. It is not simply a cycling path: it preserves an important part of Asturias’s industrial history while providing access to one of the region’s best-known wildlife conservation projects. The possibility of covering the route by bicycle, stopping at the bear enclosure and using the return service from Buyera makes it a practical activity even for people who are not regular cyclists.
Overall, the Senda del Oso is an accessible way to explore the valleys of central Asturias and discover the history of its former mining railway. The tunnels, bridges and gorges provide a clear impression of the original railway infrastructure, while the Cercado Osero of Proaza adds an important wildlife conservation element to the route. The story of Paca and Tola remains an essential part of its identity, while Molina represents the present-day continuation of the enclosure’s work. For visitors wishing to explore the trail by bicycle, the rental service at Entrago with return transport from Buyera is a particularly convenient option.