We report on under-ice profiling float observations of cold, dense, and oxygenated bottom layers on the continental slope of Mac Robertson Land (60°–72°E) in East Antarctica. This bottom layer water mass, with potential temperature in the range −1.8°C < θ < −0.4°C, is identified as modified shelf water. It is a downslope variety of dense water formed on the Antarctic continental shelf in winter and plays an important role in ventilating the deep Southern Ocean. The seasonal evolution of its thickness and density follows the sea ice cycle of growth and decay, reaching a maximum in October–November. The characteristics and location of this modified shelf water are similar to Cape Darnley Bottom Water, thus suggesting the same primary source in the Cape Darnley polynya region. These float data support recent results that the continental shelf along Mac Robertson Land is a significant source of dense waters in East Antarctica.