Researchers maintain a catalogue of satellite images of the frozen Antarctic seas that dates back to the late 1970s. While the northern Arctic has lost an average of 20,800 square miles a year in that time, NASA says, the Antarctic has gained an average of 7,300. On an average year, Antarctica's sea ice maxes out at about 7.23 million square miles. On September 20, it peaked at 7.78 million--the most ever. Satellite images show how much of the ocean's surface freezes, but ignore other factors like ice thickness and volume. Massive, ancient chunks of ice have disappeared into the ocean even as this temporary, seasonal, and relatively thin cover has grown.