A layer of diamonds up to 18 kilometres (11 miles) thick could be tucked beneath the surface of the smallest planet in our solar system.
According to new research from international scientists, the diamonds might have formed soon after Mercury itself came into existence around 4.5 billion years ago, during a period of intense heat and pressure.
At this time, it's believed Mercury was formed fully rich in carbon and had a crust of graphite floating over a deep magma ocean.
With such high carbon content, Yanhao Lin, a staff scientist at the Center for High Pressure Science and Technology Advanced Research in Beijing and co-author of the study said it 'made me realise that something special probably happened within its interior.'
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What the team of researchers from China and Belgium did was recreate and simulate that extreme environment in an experiment using an anvil press machine.
These machines are used to study how materials behave under extreme pressure and produce synthetic diamonds.
'It’s a huge press, which enables us to subject tiny samples at the same high pressure and high temperature that we would expect deep inside the mantle of Mercury, at the boundary between the mantle and the core,' said Bernard Charlier, head of the department of geology at the University of Liège in Belgium and the study's co-author.
To mimic Mercury's early interior, the team placed a synthetic mixture of elements like silicon, titanium, magnesium, and aluminium inside a graphite capsule.
They then exposed the capsule to pressures nearly 70,000 times greater than those on Earth's surface and temperatures up to 2,000 degrees Celsius (3,630 degrees Fahrenheit), replicating conditions that likely existed near Mercury's core billions of years ago.
After it was all done and melted, the scientists looked at changes in the chemistry and minerals under an electron microscope and to their shock, the graphite had turned into diamond crystals.
The findings were published on June 14 in the journal Nature Communications.
Mercury is the second densest planet after Earth. Its large metallic core takes up 85% of Mercury’s radius.
Discoveries like these can give us a deeper understanding of what's going on below Mercury's surface, uncovering its mysteries and the evolution of planets.