First attempt: Project Mohole, Baja California In the 1950s, US scientists tried to drill down to get mantle samples, after a group of geologists who met in a drinking club decided it was worth a shot. They called it Project Mohole. Drilling began off the coast of California, but the project was cancelled by a young Donald Rumsfeld after reaching only 183 metres from the surface



(Image: Fritz Goro/Time Life Pictures/Getty Images)

Deepest on land: Kola Superdeep Borehole, Russia Since then, drilling has reached much deeper. The world's deepest borehole on land descends 12,262 metres from the surface in a remote region of north-west Russia. Still, it remains only around a third of the way to the mantle because the continental crust is tens of kilometres thick.



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Longest borehole: Sakhalin-I borehole, Russia In 2011, the oil company Exxon Mobil staked a claim for the world's longest borehole, at 12,345 metres. It wasn't drilled vertically downwards, though. The aim was to acquire oil closer to the surface rather than reach down to the mantle.



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Closest to mantle: Hole 1256D, Costa Rica The closest that drills have got to the mantle is Hole 1256D, bored by scientists off the west coast of Costa Rica. It reaches 1507 metres below the ocean floor. It is the closest to the mantle because the crustal thickness there is estimated to be only 5 to 5.5 kilometres, thinner than most other places on Earth.



It's not the deepest hole in the ocean crust, though – that prize goes to another hole called 504B in the eastern Pacific, which plunges 2111 metres below the sea floor in thicker crust.



(Image: Credit Benoît Ildefonse, CNRS-Université Montpellier II & IODP)