The discovery of nearly identical sea creatures on either side of a now-solid Antarctic ice sheet — 1,500 miles wide and over a mile thick — points to an open ocean passage there as recently as 125,000 years ago. The new evidence adds to geologic clues indicating the West Antarctic Ice Sheet has collapsed at least once in the last million years, and could do so again in a warmer climate. The complete collapse of the West Antarctic Ice Sheet would raise global sea level by 11 to 16 feet.