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

Inaki Relanzon / Potholer travelling through flooded underground passageways in small inflatable boat, Cueva Cuvera, Lago de Entrada, Ason, Cantabria, Spain

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

Inaki Relanzon / Potholer taking photograph, covered in mud, Molino de Aso cave, Huesca, Spain, October 1992

Inaki Relanzon / Potholer about to lower himself down through pothole to caves below, Avenc del Passant, Garraf-Ordal NP, Barcelona, Spain

Inaki Relanzon / Potholer walking on mounds of salt crystals in cave, Cave Lachambre, Ria, Conflent, Pyrenees, France

Inaki Relanzon / Potholer in cave admiring stalagmites, Cueva Coventosa, Ason, Cantabria, Spain

Inaki Relanzon / Potholer in cave admiring stalagmites and stalactites, Cueva Coventosa, Ason, Cantabria, Spain

Doug Allan / Scientist in ice cave, Signy Island, Antarctica

Roberto Rinaldi / Person climbing inside a cave carved by a river flowing inside the Perito Moreno glacier during summer, Patagonia, Argentina.

Roberto Rinaldi / A person standing at the end of a tunnel in a glaciar formed by an ice melt glacial river, Perito Moreno glacier, Los Glaciares National Park, Patagonia, Argentina

Roberto Rinaldi / Speleologist ice climbing down deep cave in the Perito Moreno glacier, Patagonia, Argentina