It's the beginning of spring in the Southern Hemisphere - the time of year when the so-called "ozone hole" forms over Antarctica.

Now, this isn't a "hole" in the atmosphere, it's just a reduction in the amount of one gas - ozone - 10 to 30 miles up. This decrease is linked to human-made chemicals called chlorofluorocarbons - CFCs for short. The chlorine in CFCs destroys ozone, and the bitter cold high above Antarctica helps the process along.

This year, the ozone hole was the largest ever observed - three times the area of the United States. In these striking images from NASA, the ozone hole is the bluish-purple oval-shaped area centered over Antarctica. The huge size of the hole this year is disappointing, because concentrations of CFCs have actually leveled off in recent years, and the long-term prognosis is for the ozone hole to get smaller. This year's record-setter clearly shows that we still have a long way to go before this environmental problem is fixed.

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