Meteorites crashing down upon the Earth’s surface are relatively common, but a team of researchers from Monash University in Australia and Imperial College London in the U.K. may have just made a rare discovery. According to a report from Popular Mechanics, scientists were stunned to find the oldest meteorites ever known to have landed on Earth – at a whopping age of 2.7 billion years.

Geologists unearthed nearly 60 meteorites that were preserved in limestone in Western Australia between 2014 and 2015. Each sample is no larger than a grain of dust, but researchers say these tiny rock fragments can offer new insights into the makeup of Earth’s early atmosphere.

According to lead researcher Andrew Tomkins, a geologist from Monash University in Melbourne, “As geologists, we’re taught that the earth had no oxygen in its atmosphere before 2.3 to 2.4 billion years ago. [These meteorites] traveled through an upper atmosphere with almost modern day Earth-like amounts of oxygen.”

The discovery of the tiny fragments offers an important realization to scientists studying the development of the atmosphere upon which we rely today. As meteorites entered the atmosphere, their chemical makeup was altered by the friction created by the air particles suspended high above the surface of the planet. As the space rocks cooled, new molecules were formed – revealing just what was hanging around above the Earth billions of years ago.

The research team says it only took about two seconds for these “micrometeorites” to melt and cool on their way down toward Earth’s surface. Based on the team’s calculations, the meteorites likely traveled through an atmosphere that was nearly 20 percent oxygen. The key to solving this problem was found by analyzing the composition of the metal oxides present in the meteorites after they had landed on the surface of the Earth.

A press release from Imperial College London describing the details of the study can be found here.