In this paper we have applied the electrical resistivity tomography (ERT) in order to prospect and to analyse the morphological and geological subsurface of the Torcas-Cueva Mayor endokarst system (Sierra de Atapuerca). These works are essential to establish the development of the sedimentary infills where the Early and Middle Pleistocene archaeo-palaeoanthropological sites of the Sierra de Atapuerca are located. The prospecting was based on the elaboration of 15 ERT sections, which were interpreted using topographic, archaeological, geological and geomorphological data. Through this procedure we have indentified the endokarst morphologies and the main lithological groups. The latter correspond to the Upper Cretaceous limestones and dolostones (> 1500 ohm m−1) and Neogene sediments and Quaternary valley infills (< 400 ohm m−1). The endokarst structures inside the Upper Cretaceous carbonates were related to empty cavities (> 1500 ohm m−1), passages filled with speleothems (400–1500 ohm m−1and with detrital materials (< 400 ohm m−1), such as the deposits of the Dolina, Elefante and Galería sites. The analysis of these subsurface structures shows that the karstic passages present a regular south–north development, starting with subsurface faults detected in the north margin of the main valley (Arlanzón River) and finishing along the Pico valley headwaters. These passages were cut off by the Pleistocene incision of the Sierra de Atapuerca minor valleys (e.g. Propiedad valley), forming entrances to caves that were occupied by hominids and fauna from Early Pleistocene times.
