Hidden images in Atxurra Cave (Northern Spain): A new proposal for visibility analyses of Palaeolithic rock art in subterranean environments. - Intxaurbe, Iñaki; Rivero, Olivia; Medina-Alcaide, M a Ángeles; Arriolabengoa, Martín; Ríos-Garaizar, Joseba; Salazar, Sergio; Ruiz-López, Juan Francisco; Ortega-Martínez, Paula; Garate, Diego (Quaternary International, 2020)