Europe, Alpine: Figure 2. Classic alpine glaciokarst in the high Pyrenees, with bare limestone lapiaz fretted with deep karren and broken by small dolines with soil floors. (Photo by Tony Waltham)

Alpine fold mountains continue into Spain as the Cantabrians. The Picos d’Europa (see separate entry) has the highest and most spectacular karst with an amazing number of deep cave systems, but there are other significant karst sites. The Matienzo depression extends over 26 km2, largely drained into a ponor that lies 220 m below the lowest col through the perimeter mountains; the karst is underlain by numerous caves, of which >200 km have been mapped to date, with three systems each >25 km long. South of Matienzo, another karst plateau is broken by the 302 m deep Cueto shaft which continues down into large abandoned galleries and an active river passage through to the Coventosa resurgence, making a cave system 32 km long and 815 m deep. Further east, Ojo Guarena is the longest cave in Spain with a rambling maze of 99 km of mapped passages beneath a low escarpment.