The largest ozone hole is over
Antarctica
The ozone hole is caused by the effect of pollutants in the atmosphere which destroy the stratospheric ozone. During the Antarctic winter, strong circular winds form that blow around the whole continent. This is known as the "polar vortex". This isolates the air over Antarctica from the rest of the world. Secondly, special clouds form. These are called polar stratospheric clouds. Clouds do not normally form in the stratosphere and these turn out to have the effect of concentrating the pollutants that break down the ozone. By the time spring arrives and the sun comes back after the long polar night, the ozone levels are severely depleted around the Antarctic continent causing the "ozone hole". This is then followed by a particularly long period of high sunshine and long days, just to make the effect of the ozone hole worse with all that UV light around.