Studies of gravity variations at Mars have revealed dense, large-scale structures hidden beneath the sediment layers of a lost ocean. The analysis, which combines models and data from multiple missions, also shows that active processes in the Martian mantle may be giving a boost to the largest volcano in the solar system, Olympus Mons. The findings have been presented this week at the Europlanet Science Congress (EPSC) in Berlin by Bart Root of Delft University of Technology (TU Delft).