This research has been funded through the GEOCAVE project(MAGRAMA-580/12, OAPN) and a fellowship granted to D. Ballesteros by the Severo Ochoa Program (FICYT, Asturias regional Government). We acknowledge the support of the administration and staff of the Picos de Europa National Park and the Federación d' Espeleoloxía del Principáu d' Asturies. We are indebted to Dr. Merino-Tomé and Dr. Bahamonde (University of Oviedo) for their help in understanding the sedimentology of the limestone from the Picos de Europa. We are also grateful to L. Adrados (GEOLAG) and to the cavers (G. Cañón, J. de Felipe, A. Caldueño, R. Martínez, R. Fernández, G. Sendra, G. Chôrvot, J.A. Carbajal, R. Sanmartino, J.R. Saiz, C. Sedano), and caving groups (Grupo Espeleológico Polifemo, Société Suisse de Spéléologie, Asociación Deportiva GEMA, Grupe Spéléologique du Doubs, SIS del Centre Excursionista de Terrassa and GES Montañeiros Celtas) for their helpful assistance. Finally, the authors acknowledge the suggestions provided by two anonymous reviewers that contributed to improving the first version of the manuscript.