The aim of this work is to investigate the causes of the El Descargador tailings dam failure, at the mine district Cartagena–La Unión (SE Spain), in October 1963. Dam stability back analyses have been carried out by applying a geotechnical and geophysical approach. The failure occurred in the form of several landslides in five different points along the dam structure. The rise in the pore pressure and the steeped slopes of the tails, scaling up to 40° in some sectors, were the main causal preparatory factors. Here we propose that static liquefaction is the most plausible cause of the tailings dam flow failure. The presence of sand dikes and sand volcanoes with atypical stratigraphic architecture, both in the lagoon and at the surfaces exposed in the landslide areas, as well as the evidence of conspicuous sand fraction on the surface support the occurrence of the liquefaction processes. Major landslides were located near the drainage pipe and the flow directions were controlled by its position. Our results reveal that the liquefaction processes were triggered and aggravated by the poor drainage capability of the tailings dam structure.