The Mars Express spacecraft radar profiles collected data between May 2012 and December 2015 contain evidence of liquid water trapped below the ice of the South Polar Layered Deposits. Bright subsurface reflections are evident within a well-defined, 20-kilometre-wide zone centred at 193°E, 81°S. Analysis of the radar signals shows that this bright feature has a high permittivity that matches that of water. We interpret this feature as a stable body of liquid water on Mars.
http://science.sciencemag.org/content/early/2018/07/24/science.aar7268