Change in ozone hole over South Pole, from 1979 (left) to 2008 (right). This pair of images show the beginning and end of a nearly 30-year series of images. The 1979 image was captured by NASA's Total Ozone Mapping Spectrometer (TOMS) instrument aboard Nimbus-7, and the 2008 image is from the Royal Netherlands Meteorological Institute Ozone Monitoring Instrument (OMI) that flies on NASA's Aura satellite. Purple and dark blue areas are part of the ozone hole. These maps show the state of the ozone hole each year on the day of maximum depth, the day the lowest ozone concentrations were measured. Prior to 1979, scientists had not observed concentrations below 220 Dobson Units (DU). Scientists use the word "hole" as a metaphor to describe the size and depth of the area over the South Pole in which ozone concentrations, thinned due to chloroflourocarbons (CFCs), drop below this historical threshold. In 1979, the ozone hole reached its maximum depth of 194 DU on September 30, not far below the historical low and mostly confined to a relatively small area centered on the Antarctic Peninsula. Almost three decades later, the ozone concentration during the 2008 Southern Hemisphere spring bottomed out on October 4 at just 100 DU. The ozone hole encompassed virtually all of Antarctica and reached across the Southern Ocean toward the tip of South America.