The phrase "ancient air" refers to the air that has been around for a very long time. It suggests an atmosphere or environment that seems old or deeply rooted in history. Full definition
Data has also been gathered from ice cores that contain ancient air bubbles. (techtimes.com)
By analyzing the chemistry of bubbles of ancient air trapped in Antarctic ice, scientists have been able to determine the composition of Earth's atmosphere going back as far as 800,000 years, and they have developed a good understanding of how carbon dioxide levels have varied in the atmosphere since that time. (climatestate.com)
This liberates ancient air bubbles that were trapped in the ice as it formed from falling snow thousands of years earlier. (scientificamerican.com)