How big is the Martian core? What is its composition? How does it compare to Earth’s core? The Nasa-led InSight mission placed the first seismometer on the surface of Mars, collecting data for four years, allowing us to develop new answers to these important questions. In this seminar I will talk about how seismic data from Mars, together with research from mineral physics and cosmochemistry, have helped us to look deep into Mars’s core. The seminar will address how the first seismically-informed models of the whole planet were built, as well as their implications for our understanding of core composition and evolution.