South polar layered deposit (SPLD) on Mars.
The Mars Express radar experiment, MARSIS, was designed to penetrate deep and it has delivered on its promise. The above figure shows the base of the SPLD at the deepest recorded point of 3.7 km. In contrast, The Shallow Subsurface Radar (SHARAD) on NASA’s Mars Reconnaissance Orbiter designed as a high-resolution radar for a maximum penetration of 1 km has difficulty detecting the SPLD base. The two complementary instruments work together to discover hidden martian secrets.