Exploration of the Martian subsurface is evolving  rapidly, driven by multiple extended missions, preparation for Moon-to-Mars, and prioritization of the ExoMars rover and human exploration as outlined in the Decadal Survey. Maximizing scientific return through interdisciplinary collaboration is more essential than ever. This exploration follows four key themes: 1) physical, chemical, and thermodynamic properties of Mars’ interior, 2) the distribution and stability of liquid water and ice inventories using novel and Earth-based methods, 3) the provenance, cycling, and isotopic composition of bio-relevant trace gases, and 4) identification of habitable “refugia environments” where conditions could feasibly support an extant biosphere. For the ninth “New Mars Underground”, we invite abstracts that advance these themes, particularly those that integrate remote sensing, modeling, terrestrial analogs/synthetic samples, and mission concept development to illuminate the scientific value that the Martian subsurface represents for planetary evolution, comparative planetology, and development of habitable environments.