The establishment of a permanent ice sheet on Antarctica near the Eocene-Oligocene boundary (approximately 34 million years ago) is closely linked to the opening of the Drake Passage between South America and Antarctica, which allowed the formation of the Antarctic Circumpolar Current (ACC); this current thermally isolates Antarctica by preventing warmer subtropical waters from reaching the continent, a key factor (alongside declining atmospheric CO2 levels) in triggering and sustaining large-scale glaciation on the continent.