Washington -- The largest ozone  hole ever observed has opened up over Antarctica, according to the scientists of the US National Aeronautics and Space Administration (NASA). They believe it is a sign that ozone -- destroying gases produced years ago are just now causing the largest quantities of ozone to disappear. This year's South Pole ozone hole spreads over about 28.5 million square kilometers, an area three times larger than the landmass of the prefix = st1 /United States. Pictures of the hole have been offered by NASA. The hole appears as a giant blue mass, totally covering Antarctica and stretching to the southern tip of South America. "The last time the ozone hole was close to this size was in 1998, when it spread over about 27.2 million square kilometers," NASA said. Paul Newman, who works with NASA's Total Ozone Mapping Spectrometer (TOMS) instrument on a NASA satellite, said ozone watchers had expected a big hole this year, but not this big. The Antarctica ozone hole, first observed in 1985, is caused by the depletion   of Earth protecting ozone by human-made chemicals such as chlorofluorocarbons, known as CFCs. "Even though these chemicals were not allowed to use from the beginning of 1987, they remain in the atmosphere and will continue to do so for years," Newman said. "This year's large hole may have been caused by a change in a swirling high-level air current over Antarctica, which circles the area and contains the zone hole," Newman said. The text is mainly about   _   .
the discovery of the largest ozone hole the discovery of the Antarctica ozone hole the history of the Antarctica ozone hole the size of the largest ozone hole