Investigators have measured ozone levels in the atmosphere since the late 19th century, first from the ground and then within the atmosphere, aided by sophisticated airborne devices. Some of the earliest data showed that the "natural" level of ozone close to the ground at one measuring post in Europe roughly a century ago was about 10 parts per billion. Today the ground-level concentrations in Western Europe are from to four times higher. Abundances more than 10 times higher than the natural level are now often recorded in Western Europe, California the eastern and Australia.