The Martian crustal subsurface encompasses a wide range of environments at depths from ~centimeters to kilometers. These environments are relatively unexplored but are of enormous interest for planetary science. Recent results, e.g., methane fluctuations, radar data that are consistent with liquid subsurface water, and ongoing debates on RSL, all point to dynamic subsurface environments. We invite contributions that address the nature and diversity of Mars crustal subsurface environments (modeling, experiments, observations) or develop the tools/missions for exploring them (sounding, access, in situ analysis). We are particularly interested in contributions that advance our understanding of how the subsurface changes with geographic location and depth, in respect to: volatiles such as brines, ices, clathrates, salts, methane and oxidants, the potential for extant life and the preservation of signs of extinct life, the redox potential of past and present environments, and the technologies/mission concepts that enable such subsurface exploration.