Abstract: The distribution and concentration of subsurface water ice on Mars are key constraints on past climate and the processes that led to the ice’s deposition. We investigate the subsurface of Arcadia Planitia by measuring the depth of terraces in simple impact craters and mapping a widespread subsurface reflection in radar sounding data. Our mapping of these data sets has allowed us to constrain the thickness, latitude range and volume of an extensive subsurface layer. We have also constrained the composition of the layer by combining these data, and results are consistent with a widespread deposit of excess ice in the northern mid-latitudes of Mars. Understanding the conditions that formed and preserved this thick, extensive layer of ice will improve our understanding of the Martian climate system and the deposit may also serve as a resource for future human exploration to Mars.