IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
Has anyone ever drilled all the way through Earth’s
Earth is mostly a huge ball of molten rock, covered in a thin and fragile solid crust on
which we live. To examine the interior of our world, we need only drill through that crust.
But it’s not exactly simple ….
The Kola Superdeep Borehole, drilled by the Soviet Union, reached a depth of 40,230 feet (12km). Despite
this incredible depth, the bore reached only one third of the way through the crust. But even at that depth,
things got very, very strange ….
Let’s start this answer with some numbers. The
Earth has a diameter of 7,917.5 miles (12,742km).
The crust varies in thickness from about 3 to 6
miles (5 to 10km) on the seaﬂoor, to 20 to 30 miles
(32 to 48km) thick under the continents. In other
words, compared to the planet as a whole, the crust
is very thin indeed.
Most of the Earth is made of a solid but hot and
malleable shell about 1,800 miles (2,900km) thick,
called the mantle. It takes up about 84 percent of
the Earth’s volume. The core of the Earth is made
of iron and nickel and has two layers: a liquid outer
layer, and a solid inner layer. The core makes up 15
percent of the planet.
That means the crust is just 1 percent of the
total mass of the Earth. But humans still strug-
gle to penetrate it to any signiﬁcant depth.
The closest we’ve come is the Kola Super-
deep Borehole. This drilling project in the
former Soviet Union, on the Kola Peninsula east
of Norway, managed to get 40,230 feet (12km)
into the continental crust.
The eort was immense. Nineteen years of
drilling, multiple drill bits, endless engineering
challenges, broken drills, secondary shafts …
and at the end of it all the project had made it
barely one third of the way through the crust.
Part of the problem was intense heat. Scien-
tists had predicted the crust would be as hot as
212°F. But in fact, the rock was 356°F, and only
getting hotter. After reexamining the numbers,
the drill team realized that if they were to reach
their target depth of 49,000 feet (15km), it
would mean working at 570°F. Unfortunately,
at that temperature, the drill bit itself would no