The classic Ordesa walk, and one of the finest day hikes in Spain. The route follows the Arazas river upstream through the valley floor, passing through beech and fir forest with the canyon walls rising sheer on both sides, until it opens into the Soaso cirque and the Cola de Caballo waterfall drops 70 metres into a pool below Monte Perdido.
The path out and back is about 18 km and takes five to six hours at a steady pace. The gradient is gentle for the first two-thirds; the final push up to the waterfall is steeper. Start early in summer — the valley can fill up by mid-morning, and the trail is better in cool air.