Marine ice is mainly formed by the accumulation and consolidation of ice platelets beneath anice shelf. Layers of marine ice beneath ice shelves provide strong evidence of a vigorous andwidespread sub-ice shelf ocean circulation. This circulation is primarily thermohaline driven andfollows an "ice pump" process, which melts ice in great depth and releases ice crystals inshallower areas. A marine ice body influences both the mass balance and flow of an ice shelf, itsicebergs, and the by-product of its formation, Ice Shelf Water, which has a major influence onthe oceanography of the Southern Ocean. Thus far, only two major Antarctic ice shelves, theAmery and Ronne ice shelves, have been shown to be underlain by significant marine ice bodies.We present a multi-disciplinary approach that indicates the Filchner Ice Shelf is also underlain bysuch a body of marine ice. Radio echo sounding, ice and ocean modeling, and remote sensingindicate a marine ice body of > 350 km^3 located near the northwestern part of the ice shelf,extending over an area of > 4000 km^2. The equilibrium (steady-state) mean basal balance iscalculated to be about 0.35 m(ice)/a of melting. Inter-comparison of different data sources providesadditional insight into the behavior of Filchner Ice Shelf. The results show the complexity andimportance of basal ice accretion for the evolution of ice shelves in general.