The Deboyu cave, in Caso, inside the Redes Natural Park, is a karst gallery about 200 metres long through which the Nalón river runs. It has been a Natural Monument since 2003 and you cannot go in, because it is the park's most important bat refuge; both of its mouths can be seen from outside.
A few kilometres after its source, the Nalón river does something that looks like a trick: it goes into a limestone mountain and comes out about 200 metres further down. The tunnel it has carved is the Deboyu cave, a Natural Monument since 2003, inside the Redes Natural Park, in the municipality of Caso. You see it from outside, you reach it on a short walk and you do not go in, for a good reason: it is home to the park's bats.
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A river that crosses a mountain
The official fact sheet of the protected area describes it as a karst gallery about 200 metres long through which the Nalón river runs, at 544 metres above sea level, next to the village of Les Llanes, which Turismo Asturias spells Les Yanes. Over thousands of years the water has dissolved the limestone until it found a way through the ridge, while from outside the mountain is still whole.
It was declared a Natural Monument by Decree 39/2003 and protects 2.6 hectares. It lies inside the Redes Natural Park, which covers the municipalities of Caso and Sobrescobio, was declared in 1996 and has been a Unesco Biosphere Reserve since 2001.
Where to see the way in and the way out
The road that climbs from Campo de Caso towards the Tarna pass runs parallel to the river, and both mouths can be seen from it. According to the official fact sheet, the entrance is visible past the Puente Colorau and the exit from the Puente Deboyu. Escapada Rural says that, past Campo de Caso and just before the second tunnel, a turning on the right signposted cueva Deboyu leads to the picnic area.
The easy walk and the long one
The simplest starts at the car park next to the Puente Colorau and ends at a picnic area by the river, with wooden tables and shade. It is 1.25 kilometres there and back, linear and fine for small children, according to Escapada Rural, which publishes the track.
For those who want to walk more, the same source describes a loop of about 9.3 kilometres and about 750 metres of climbing, rated difficult, that rises through chestnut, oak and beech woods to Peña Blanca. Autumn, with the forest turned gold, is when it looks its best.
Why you cannot go in
The official fact sheet calls it the most important refuge for cave-dwelling bats in the Redes Natural Park. Five species use it: a mixed colony of Mediterranean horseshoe bats and Daubenton's bats, and at least three more, the greater horseshoe bat, the common pipistrelle and the Schreibers' bat, the last listed as of special interest in the Regional Catalogue of Threatened Species.
Escapada Rural explains that entry is forbidden because it is a breeding and hibernation colony, and Turismo Asturias notes that most of the reserve's bats nest there. You look from outside, quietly, and that is enough.
The river around it
Outside the cave, the Nalón runs between alder groves. The official fact sheet mentions brown trout on this stretch and, as signs of clean water, otters and Pyrenean desmans, as well as river birds such as the dipper and the kingfisher.
It is the same river that later flows down through Laviana, San Martín del Rey Aurelio and Langreo. According to Wikipedia it rises at the Nalona spring, at the Tarna pass, at 1,443 metres; it runs 153 kilometres through twelve municipalities and reaches the sea at the Pravia estuary. Deboyu is one of its first episodes, and one of the strangest.
When to go and what else is nearby
You can go all year round. Escapada Rural recommends late spring, when snowmelt swells the river, and autumn, for the colour of the woods. In El Campu, close by, is the park's Centro de Recepción e Interpretación de la Naturaleza, the place to ask about the state of the trails.
On the last Saturday of August the Certamen del Quesu Casín is held there, a festival of tourist interest of the Principality, with a producers' market and a competition for the cheese with Protected Designation of Origin; in 2026 it was on 29 August, its 36th edition.
Checked
Facts checked in October 2026 against: the official fact sheet of the Cueva Deboyo Natural Monument from the Principality of Asturias (2010 copy), Turismo Asturias (Cueva Deboyo and Certamen del Quesu Casín), Vivir Asturias (Cueva Deboyo Natural Monument), Escapada Rural (walk to the Deboyu cave) and Wikipedia (Nalón river and Redes Natural Park).
Quick questions
Is there an easy walk to see the Deboyu cave?
Yes: it starts at the car park next to the Puente Colorau and reaches a picnic area by the river; it is 1.25 kilometres there and back, fine for small children.
Can you go inside the Deboyu cave?
No. Entry is forbidden because it is a breeding and hibernation colony for five species of bats.
When is the best time to go?
You can go all year round; late spring, when snowmelt swells the river, and autumn, for the colour of the woods, are recommended.