Even though the world is 4.54 billion years old, the rocks on it are not. The many processes that shape the crust and the higher levels of our planet change these rocks constantly: they form, they grow, they break, they turn to dust, and they turn back into rocks.

Some of the oldest rocks on Earth have persisted for about 4 billion years, but researchers believe they have now discovered evidence for something much older than that. Superdeep diamonds found in Brazil hint at a rock reservoir located at least 410 kilometers (255 miles) below sea level that could be a mere tens of millions of years younger than the Earth itself. The study is published in the Journal Science.

During its formative years, the Earth experienced violent volcanism and tremendous changes in its geology. It also collided with a Mars-size object, which threw so much material into orbit that half of it turned into the Moon we all know and love today. There’s now very little left of Earth’s original rock, but the reservoir appears to have been undisturbed since the planet's formation.

Since the 1980s, scientists have found hints that something deep inside the mantle was much older. When they compared the amount of helium-4 and helium-3, they found the ratio of these elements were closer to what we see in ancient meteorites rather than what we observe on Earth today. Scientists have also been looking at chemical clues trapped in superdeep diamonds and found support for the idea of an ancient unchanged material deep beneath the Earth's crusts. 

“Diamonds are the hardest, most indestructible natural substance known, so they form a perfect time capsule that provides us a window into the deep Earth. We were able to extract helium gas from twenty-three super-deep diamonds from the Juina area of Brazil. These showed the characteristic isotopic composition that we would expect from a very ancient reservoir,” research leader Dr Suzette Timmerman, from the Australian National University, said in a statement.

“From the geochemistry of the diamonds, we know that they formed in an area called the transition zone, which is between 410 and 660 kilometers below the surface of the Earth. This means that this unseen reservoir, left over from the Earth's beginnings, must be in this area or below it.”

The research is a first step in understanding what the reservoir is like. It is still unclear if there is a single large reservoir or multiple small ones. Its chemical composition is also unknown, but having possibly found the oldest undisturbed material on Earth is definitely intriguing.