Instrument on the Mars Express spacecraft that is the first sub-surface sounding radar system used to explore a planet. Developed by a joint Italian/US team, MARSIS employs two 20-metre antennae capable of characterizing the subsurface structure of Mars to a depth of a few kilometres, provide altimetry and roughness data, and make ionospheric measurements. To date MARSIS data have revealed that Mars has an older, craggier face buried beneath its surface, including some impressive impact cratering that lies buried beneath much of the smooth, low plains of the planet’s northern hemisphere. The data also revealed that the southern pole of Mars forms an ice dome that stretches 1 000 km in diameter, renewing the possibility that the planet might have once been able to sustain life. In October 2016, MARSIS data revealed that the planet’s surface also has numerous pockets of strong magnetism locked up within its crust (remnants from its earliest days), suggesting that the Mars we see today was different from its past.