Here's the original paper (no paywall, and apparently not linked in the BBC article): http://science.sciencemag.org/content/early/2018/07/24/science.aar7268 ht

Here's the original paper (no paywall, and apparently not linked in the BBC article): http://science.sciencemag.org/content/early/2018/07/24/science.aar7268 http://science.sciencemag.org/content/early/2018/07/24/scien... A couple of highlights: My first thought was why couldn't this be some other liquid like CO2: We examined other possible explanations for the bright area below the SPLD (supplementary text). For example, a CO2 ice layer at the top or the bottom of the SPLD, or a very low temperature of the H2O ice throughout the SPLD, could enhance basal echo power compared with surface reflections. We reject these explanations (supplementary text), either because of the very specific and unlikely physical conditions required, or because they do not cause sufficiently strong basal reflections (figs. S5 and S6). Although the pressure and the temperature at the base of the SPLD would be compatible with the presence of liquid CO2, its relative dielectric permittivity is much lower (about 1.6) (28) than that of liquid water (about 80), so it does not produce bright reflections. Why liquid and not solid water? ... Perchlorates can form through different physical and/or chemical mechanisms (30, 31) and have been detected in different areas of Mars. It is therefore reasonable to assume that they are also present at the base of the SPLD. Because the temperature at the base of the polar deposits is estimated to be around 205 K (32), and because perchlorates strongly suppress the freezing point of water (to a minimum of 204 and 198 K for magnesium and calcium perchlorates, respectively) (29), we therefore find it plausible that a layer of perchlorate brine could be present at the base of the polar deposits. ... This is a similar principle (freezing point depression) to that used in de-icing roads. And maybe the best part of all - there may be other smaller but currently undetectable pools: ... The limited raw-data coverage of the SPLD (a few percent of the area of Planum Australe) and the large size required for a meltwater patch to be detectable by MARSIS (several kilometers in diameter and several tens of centimeters in thickness) limit the possibility of identifying small bodies of liquid water or the existence of any hydraulic connection between them. Because of this, there is no reason to conclude that the presence of subsurface water on Mars is limited to a single location. ...