The Bardenas Reales are 420 square kilometres of clay badland in southern Navarre, and nobody lives in it. Too little rain falls for plants to hold the ground, so the carving is done by the rare cloudburst, which cuts ravines, mesas and lone towers out of the flat. Castildetierra, the one everybody comes for, stands alone on the clay: a fluted cone that widens downwards and carries a slab of hard sandstone on its head. That slab is the only reason it is still standing, and the park seals the cracks in it to stop it shearing away. A wooden fence keeps you at the foot. The last hour of the day is what you come for. Low sun runs in sideways, the ridges take hard edges and the hollows fill with shadow.
Gravel tracks run for tens of kilometres between the cabezos, hard enough for a bicycle or an ordinary car, with no shade between you and the next horizon. Rain closes them: wet clay turns to paste. Where the long plateaus wall in the pale flats, their flanks are grooved from top to bottom like organ pipes, and after rain you can pick out the channels the last storm cut. Little grows, and yet twenty-four kinds of bird of prey hunt here, along with great bustards, sandgrouse and stone-curlew. Some 250 pairs of Dupont's lark live out on the steppe, heard at dawn far more often than seen. In the middle of it all, 22 square kilometres are not park but an air force bombing range, in use since 1951. The rest belongs to twenty-two owners at once — nineteen towns, two Pyrenean valleys and a monastery — who hold the desert as common land. Nobody may settle here or own a plot, so the ground is used rather than lived in. The using has a date. Every September a rocket goes up at eight in the morning, and a dozen shepherds open the gate in the fence called El Paso. Some eighty thousand sheep come down off the Pyrenees to winter on the clay, and walk back up in June.