The biggest ozone hole in the history of the Earth was created not by man but by Nature 251 million years ago, according to researchers.

The hole circled the planet around the Equator and extended as far north as central Portugal and as far south as southern Argentina. It was created by volcanic eruptions in Siberia that led to the biggest mass extinction of life and helped to hasten the death of countless species of plants and animals.

Scientists calculated that the ultraviolet radiation penetrating the hole was up to six times more intense than that coming through the hole created by man through the use of CFCs and other chemicals.

Once the chemicals that destroyed the ozone layer in the Permo-Triassic period had stopped pouring into the atmosphere, it took only about a decade for the hole to heal. The ozone-destroying chemicals man put into the atmosphere are more long-lasting.

Researchers, led by Professor David Beerling at the University of Sheffield, published their findings in Philosophical Transactions of the Royal Society.

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