Mars may possess a global sub-surface groundwater table as an integral part of its current hydrological system. However, the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) onboard the Mars Express (MEx) spacecraft has yet to make a definitive detection of such a body of liquid water. In this work, we quantify the conditions that would allow a detection of a deep aquifer and demonstrate that the lack of radar detection does not uniquely rule out the presence of such a body. Specifically, if the overlying crustal material has a conductivity above ∼10⁻⁵ S/m (equivalent to a loss tanget of 0.008), a radar echo from an aquifer could be sufficiently attenuated by the intervening medium to prevent its detection by MARSIS. As such, the lack of direct detection by MARSIS–a “null result”–does not rule out the possibility of the water table's existence.