Inaki Relanzon / Potholer touching a stalagmite whilst exploring the Graller del Boixaguer cave, Montsec mountains, Pyrenees, Catalonia, Spain, May 2012. Model released.

Inaki Relanzon / Potholer exploring the Graller del Boixaguer cave, Montsec mountains, Pyrenees, Catalonia, Spain, May 2012. Model released.

Inaki Relanzon / Silhouette of potholer in underground cave system surrounded by ice formations, Sala d'Entrada, Gruta helada de Casteret, Ordesa National Park, Spain.

Michael Pitts / BBC Producer Neil Nightingale in a limestone tunnel, Rock Islands, Palau.

Inaki Relanzon / Potholer in cave admiring early formation of stalactites, Cantabria, Spain

Inaki Relanzon / Potholer silhouetted at exit to underground cave system, Huesco, Spain

Inaki Relanzon / Potholer being lowered down into underground cave system, Boca del Mortero de Astrana, Ason, Cantabria, Spain, February 1998

Inaki Relanzon / Potholer wallking along narrow underground passage, Cova Lachambre, Ria, Conflent, Pyrenees, France

Inaki Relanzon / Potholer in underground cave system beside ice formations, Sala d'Entrada, Gruta helada de Casteret, Ordesa NP, Spain, August 1995

Inaki Relanzon / Potholer emerges from underground passage into cave with stalactites, Cova Lachambre, Ria, Conflent, Pyrenees, France

Inaki Relanzon / Potholer in underground cave system wading through deep water, Escudin, Huesco, Spain

Inaki Relanzon / Potholer sitting in cave admiring stalactites, Avenc dels Topografs, Garraf-Ordal NP, Barcelona, Spain