Canyon carved in the limestone with breathtaking scenery. The approach path shows extraordinary views of giant rocky arches, pyramids, caves, pinnacles, and 250 meters below the river flowing. During summer, the descent doesn't offer any difficulty, due to a certain lack of water. The part of "los oscuros" is extremely beautiful and the descent becomes a quite long underground descent. Birds of prey always looking out.

Access: from Barbastro follow the indication for Huesca and after about 15 km take the road on the right heading for Bierge and Rodellar. 30 km far from this junction there's Rodellar, a tiny village with houses built over the ravine of the Barranco. Park at the parking area at the end of the road.

Approach: just after the little square of the village there's a track descending on the right, running towards a group of houses detached from the rest of the village. By these buildings a steep path descends towards the bottom of the canyon and reach the water near a little free-climbing equipped cliff. Go up the river following a well marked path on the right side of the river. A few minutes after the confluence of the Barranco de la Virgem with the Mascun (15’) the path crosses the river and in a while reaches the Fuente de Mascùn, a big source. The path crosses again the water and comes back on the right bank and in a few minutes disappears under the stones. Pass over the Garganta de Fornocal, a canyon flowing into the Mascun on its right side, and at about 45’ of walk from Rodellar a steep path (marked by two evident heaps of stones) rises up along the right river bank, for about 30'. Turn right at the only road junction of the track, reach the canyon cliff top and go on for 30' more until the deserted village of Otin. Pass over the village following the indications for Letosa and 30' after, next to a sharp left bend, take a path on the right descending towards a thicket. After about 10’ more a little path on the right, marked by two heaps of stones, leads into the undergrowth and in about 15’ reaches the bed of the Mascun, which must be followed for other 15 minutes, until the first bolt.

Coming back: follow the river going back to Rodellar, running back the course covered in the last hour on the bottom of the canyon.