The search for liquid water on Mars is one of the primary goals for the continued exploration of the planet. From observations of landforms at increasingly high resolution, to measurements of the thermophysical properties of the surface materials, and finally investigations by landers carrying suites of instruments, searching for evidence of liquid water has been a consistent mission parameter since the early days of Martian exploration. Today, Mars is a frozen hyperarid desert. The largest reservoirs of water are the polar caps, where water is stored as ice. Even though fluvial morphologies and lacustrine deposits have been identified, attesting to a warmer and wetter planet in the distant geological past, no liquid water exists presently on the Martian surface. Thus, exploration for liquid water focuses on the planet subsurface. The layered ejecta of impact craters indicate that ice or other volatiles were present underground at the time of impact, and the detection of hydrated minerals usually associated to circulation of warm water at shallow depths, further suggests that liquid water may exist beneath the surface. The ground penetrating radar MARSIS on board Mars Express, has been probing the Martian subsurface since 2005. In 2018 [1] and 2021 [2], the MARSIS science team detected bright reflections from the base of the south polar cap, 1.5 km below the surface. The team concluded that the source of the reflections was liquid briny water. Alternative hypotheses were subsequently proposed by other groups [3-5], but in all cases the MARSIS team demonstrated, through laboratory experiments and modelling [6,7], that those explanations were not consistent with MARSIS observations: the only material with physical properties consistent with the MARSIS data acquired over the bright reflection region, is liquid salty water. The questions about how liquid water formed and what conditions led to its stability to the present day, are wide open. The MARSIS team is continuing to work to understand the physical, geologic and climatic processes that have led to the presence of water buried at the base of the south polar cap. Aside from leading to fundamental new knowledge about the formation and evolution of Mars, the mechanisms of preservation of liquid brines beneath icy shells are relevant to planning of experiments, design and testing of instrumentation to explore Europa and Enceladus in future deep space missions. [1] Orosei et al. (2018) Science 361(6401), 490-493. [2] Lauro et al. (2021) Nature Astronomy 5, 63-70. [3] Bierson et al. (2021) Geophys Res Lett 48(13), e2021GL093880. [4] Smith et al. (2021) Geophys Res Lett 48(15), e2021GL093618. [5] Grima et al. (2022) Geophys Res Lett 49(2), e2021GL096518. [6] Mattei et al. (2022) Earth Planet Sci Lett 579, 117370. [7] Orosei et al. (2022) Icarus 383(1), 115073.