Scientists are finding an older, craggier face of Mars buried beneath the surface, thanks to pioneering sounding radar aboard the European Space Agency’s Mars Express spacecraft. Radar echoes captured by the MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) instrument strongly suggest there are ancient impact basins buried beneath the lowland smooth plains of the Martian northern hemisphere.
Geologists Finding a Different Mars Underneath

https://www.esa.int/Science_Exploration/Space_Science/Mars_Express/Mars_Express_scientists_find_a_different_Mars_underneath