ESA’s Mars Express spacecraft has discovered several ponds of liquid water buried under the ice in the south polar region of Mars. The spacecraft’s radar instrument, MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding), revealed one underground reservoir in 2018, buried about 1.5 km below the ice. Now, taking into account more data and analysing it in a different way, three new ponds have been discovered. The largest underground lake measures about 20 x 30 km, and is surrounded by several smaller ponds. The water is thought to be very salty in order for it to remain liquid at cold temperatures.