Antarctica is the southernmost continent. Including all islands and ice shelves, it covers an area of about 13.9 million square kilometres (km2)—nearly twice that of Australia. The sea ice that surrounds Antarctica adds about another 19 million km2 at its maximum extent in September–October (Fretwell et al. 2013), diminishing to 3 million km2 in February (Parkinson & Cavalieri 2012). The annual growth and retreat of the Antarctic sea ice are one of nature’s most significant large-scale annual changes. Antarctica is Earth’s coldest, highest, windiest and driest continent, and its largest cold desert. Only about 21,745 km2, or 0.18 per cent, of the total Antarctic land mass is ice-free (Burton-Johnson et al. 2016). Antarctica also has the deepest continental shelf and is surrounded by the largest wind-driven currents, which circulate the Southern Ocean. It is the only continent that has never had a native human population.