Marine deposits at +20 ± 3 m on the tectonically stable coastlines of Bermuda and the Bahamas support the hypothesis of a partial collapse of the Antarctic ice sheet during the middle Pleistocene. Beach sediments fill a sea cave at +22 m in Bermuda, and horizontal, fenestrae-filled beds crop out on platforms at two sites as high as +21 m in Eleuthera, Bahamas. Carbonate beach sands are bound by an early generation of isopachous fibrous cement that is characteristic of a phreatic marine environment. Amino acid racemization and TIMS (thermal-ionization mass spectrometry) dates constrain the age of the deposits to between 390 and 550 ka, while proxy evidence supports a correlation with oxygen isotope stage 11. This direct geologic evidence of a 20% decrease in polar ice during the middle Pleistocene has important implications for the stability of ice sheets during warm interglaciations.