The most popular walk in the Cazorla natural park follows the crystal-clear Río Borosa upstream to its source through scenery that progresses from the pretty to the majestic, via a gorge, two tunnels and a mountain lake. The walk is about 11km each way, with an ascent of about 600m, and takes about seven hours there and back.
To reach the start, turn east off the A319 at the 'Sendero Río Borosa' sign opposite the Centro de Visitantes Torre del Vinagre, and go 1.7km. The first section of the walk criss-crosses the tumbling, beautiful river on a couple of bridges. After just over 3km, where the main track starts climbing to the left, take a path forking right (with a rickety 'Cerrada de Elías' sign at research time). This leads through a lovely 1.5km section where the valley narrows to a gorge, the Cerrada de Elías, where the path becomes a wooden walkway. You re-emerge on the dirt road and continue 4km to the Central Eléctrica, a small hydroelectric station.
Past the power station, the path crosses a footbridge, after which a ‘Nacimiento Aguas Negras, Laguna Valdeazores’ sign directs you on and upward. The path winds its way up the valley, through increasingly dramatic scenery and getting gradually steeper. After about an hour, you enter the first of two tunnels cut through the rock for water flowing to the power station. It takes about five minutes to walk the narrow path through the first tunnel (the path is separated from the watercourse by a metal handrail), then there’s a short section in the open air before a second tunnel, which takes about one minute to get through. You emerge just below the dam of the Embalse de los Órganos, a small reservoir surrounded by forested hills. Take the leftward path at the dam and in five minutes you reach the Nacimiento de Aguas Negras, where the Río Borosa begins life welling out from under a rock. Enjoy your picnic beneath the spreading boughs of a large tree here, then head back down the way you came.
Due to its popularity, it's preferable to do this walk on a weekday! Do carry a water bottle: all the trackside springs are good and drinkable but the last is at the Central Eléctrica. A torch (flashlight) is comforting, if not absolutely essential, for the tunnels.