In the twilight of the Cretaceous, 86 million years ago, a volcanic fissure in what is now South Africa rumbled to life. Below the surface, magma from hundreds of miles down shot upward as fast as a car on the autobahn — if that car were barreling through solid rock — chewing up rocks and minerals and carrying them toward the surface in a reverse avalanche.

What this looked like on the surface is lost to history, but it may have been as dramatic as the eruption of Mount Vesuvius. What it left behind was a series of carrot-shaped, igneous-rock-filled tubes under low, weathered white hills. In 1869, a shepherd’s discovery of a huge, sparkly rock on a nearby riverbank would catapult this unassuming landscape into infamy. The rock was an enormous diamond that would eventually be known as the Star of Africa, and the white hills hid what would become the Kimberley Mine, the epicenter of South Africa’s diamond rush and quite possibly the largest hole on Earth ever dug out by hand.

Thanks to the Kimberley Mine, often called “The Big Hole,” the formations where diamonds are found are now known as kimberlites. The formations are sprinkled across the globe, from Ukraine to Siberia to Western Australia, but they’re relatively small and rare. What makes them special is that their magmas come from very deep down. There are still questions about precisely how deep, but they are known to arise from beneath the bases of continents at the border of the hot, convecting mantle. Some may originate even deeper, at the transition between the upper and lower mantle.

As such, these magmas tap into very deep, very ancient rock, and they interact with other processes that occur only in the deep Earth — namely, the formation of diamonds. To crystallize plain-old carbon into hard, sparkly diamond requires great pressure, so these gems form at least 93 miles down, in the deepest layers of the lithosphere, the scientific term for the crust and relatively rigid upper mantle. Some, known as sub-lithospheric diamonds, form even deeper, down to around 435 miles. Kimberlites, on their eruptive journeys to the surface, catch diamonds and drag them into the upper crust, delivering them relatively unscathed and sometimes even containing pockets of fluid from the mantle itself.