The first section of the trail climbs from the Poncebos bridge into the gorge proper. Within twenty minutes the valley walls close in and the scale of what you are inside becomes clear. The path is wide enough to walk comfortably and the surface is good — maintained stone and compacted earth, with metal railings at the exposed sections above the biggest drops.

The tunnels are the first surprise. Short sections of the path disappear into the cliff face — drilled through the rock by the original engineers — and emerge on the other side into light and space and a view that was hidden until the tunnel delivered you to it. Bring a torch or use your phone light. The tunnels are short but completely dark at the centre.

The gorge deepens as you move east. The river below becomes increasingly distant and the walls above press closer. At several points the path narrows to a ledge wide enough for one person at a time — not exposed enough to be frightening but present enough to demand attention. These sections are where the railing earns its existence.

The midpoint of the trail is marked by a bridge crossing a side gully — a good rest stop with views both back along the path you have covered and forward into the deeper section of the gorge ahead. Eat something here. The second half of the trail to Caín is longer in feel than the first.

Caín is the village at the far end of the gorge — a handful of stone houses at the bottom of a narrow valley accessible by road from the Cantabrian side but effectively cut off from the Asturian side except by this trail. It has a bar. Order whatever they have. You have walked twelve kilometers through a canyon in the Picos de Europa and you have earned a cold drink and somewhere to sit.