ESA’s Mars Express has used radar signals bounced through underground layers of ice to identify a pond of water buried below the surface. The bright horizontal feature at the top corresponds to the icy surface of Mars. Layers of the south polar layered deposits – layers of ice and dust – are seen to a depth of about 1.5 kilometer. Image by ESA/NASA/JPL/ASI/Univ. Rome; R. Orosei et al 2018