This is a route from Posada de Valdeon to the southwest, going through at least two chasms: along the Rio Aller and near Vegacervera (see picture). In between these is a part off-road on rocky gravel roads that is about 40 kilometers long. Good driving experience is needed, or you should take a detour via Campomanes, that takes at least twice the time! And you miss the chasm of Rio Aller. You can also drive to this chasm and back the same way.