What Is the Deepest Hole in the World?

Alongside the Space Race, the mid-1900s saw another contest of engineering -- a race downward, into the Earth. Learn why the USSR created the Kola Superdeep Borehole (the deepest artificial or natural hole we know of), what we learned from it, and how this research continues in today's episode of BrainStuff, based on this article: https://science.howstuffworks.com/engineering/civil/kola-superdeep-borehole.htm

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Episode Transcript

Speaker 1 (00:01):
Welcome to Brainstuff, a production of iHeartRadio. Hey brain Stuff
Lauren Bogelbaum here. In the decades after World War Two,
the United States and the USSR locked into the space
race to orbit and the Moon, but they were also
vying to outdo each other in another race, one to

(00:22):
the center of the Earth, or at least as close
to it as possible. This resulted in the deepest hole
in the world today, let's talk about the Cola Super
Deep Borehole. Okay. In nineteen fifty eight, America launched Project Mohole,
a plan to retrieve a sample from Earth's mantle by

(00:45):
drilling to the bottom of the Pacific Ocean off of
Guadalupe Island, Mexico. With funds from the National Science Foundation,
they drilled six hundred and one feet that's one hundred
and eighty three meters into the seabed before funding for
the project was pulled in nineteen six sixty six. In
nineteen seventy, the Soviets launched their attempt, drilling into the

(01:06):
ground on the Cola Peninsula in the northwestern corner of Russia,
just off the Norwegian border, near the Barren Sea. They
were more successful, penetrating much deeper and collecting samples that
still whole scientists. If you've visited the site today and
you can it's a tourist attraction. Now, it might seem underwhelming,

(01:28):
hidden in an abandoned work site, among rotting wood and
sheets of scrap metal remains of the housing and drilling
framework that once stood. There sits a small, unassuming, heavy
duty metal cover secured into place with a dozen large
resting bolts. Underneath this bottle cap is a hole that's
just nine inches that's twenty three centimeters in diameter, but

(01:51):
remains the world's deepest man made hole. It's actually the
deepest point that we know of in the Earth. The
Cola Super Deep Borehole runs about seven point six miles
that's twelve point three kilometers into Earth's surface. For perspective,
the hole's depth is the height of Mount Everest and

(02:12):
Mount Fuji stacked on top of one another. It's deeper
than the deepest point of the Ocean, the Marianna Trench,
which dips a mirror six point eight miles or ten
point nine kilometers below the surface of the Pacific. Drilling
at Cola began on May twenty fourth of nineteen seventy
The goal was to go as far as possible, which

(02:33):
scientists at the time hoped to be about nine miles
or fifteen kilometers. By nineteen seventy nine, the project had
broken all world records for man made holes when it
surpassed about six miles or nine and a half kilometers.
In nineteen eighty nine, they reached the current depth. That's
when temperatures in the well increased to three hundred and

(02:55):
fifty six degrees fahrenheit or one eighty celsius, forcing work
to stop. Turns out that digging a hole to the
center of the Earth is a bit trickier than the
researchers expected. When drilling began at Cola, the granite near
the surface was easy going, but when drillers reached about
four point three miles or six point nine kilometers deep,

(03:16):
the layers became more dense and difficult to bore into.
Drill bits broke, and the team had to change the
direction of the drilling several times, meaning that there are
a few branches off of the central hole. The resulting
pattern looks a little like a Charlie Brown Christmas tree.
The engineers plowed on, but the deeper they went the
hotter the earth became. The temperature gradient conformed to what

(03:40):
scientists had predicted to a certain point, but then the
heat intensified. They weren't expecting anything higher than the boiling
point of water, which is two twelve fahrenheit or one
hundred celsius. When the temperature climbed eighty percent higher than that,
the rock, which was also more porous that expected, started
behaving more like a plastic that a solid, rendering drilling

(04:03):
virtually impossible. Although the team pressed on until nineteen ninety two,
they never got any deeper than what they reached. In
nineteen eighty nine. The drill site was officially shut down
and the hole sealed over in two thousand and five.
But okay, why would anyone go to the trouble of
doing this? Of course, humans dig holes for various reasons,

(04:28):
most notably for extracting resources like fossil fuels and metals.
For example, in the United States, there's the Bingham Canyon
copper mine in the mountains near Salt Lake City, the
site of a pit that extends three quarters of a
mile deep that's one point two kilometers, and Oklahoma's Bertha
Rogers gas well, which goes down about six miles or

(04:48):
nine point six kilometers and had to stop because they
hit liquid sulfur. The colibor hole was partially looking for
the presence of metals and minerals down in the layers
of Earth to help locate their deposits, but holes are
also dug in the name of science to learn more
about hazards like earthquakes and volcanic eruptions, resources like geothermal

(05:10):
heat and energy, the evolution of our planet and the
life on it, and the environmental changes of the past
to better project into the future. For the article of
this episode is based on how Stuff Works. Spoke with
doctor Ulrich Hahnes, now the Liaison officer for the International
Continental Scientific Drilling Program at the GFC Hemholtz Center for

(05:33):
Geosciences in Germany. He's visited the Colobar Hoole, browsed the
repository of core samples, and even laid hands on the
now defunct well head. He explained one example in detail
is that observations very close to an earthquake zone allow
researchers to monitor the initiation and propagation of even the

(05:55):
tiniest earthquake. In response to stress and strain We want
to recover these near field physical, chemical, and mechanical data
to fundamentally understand these processes that cannot be simplified in
lab experiments or computer models. In nineteen seventy, when work
on the Colo Peninsula began, the idea that the Earth

(06:18):
has layers made up of different materials in different conditions
was relatively new. Until the late eighteen hundreds, scientists believed
that Earth's composition was the same from crust to core.
But as the technology advanced to measure earthquakes, you know,
seismic waves moving through the Earth, researchers discovered something bewildering.

(06:41):
There was a sudden increase in the velocity of seismic
waves as they passed from what we now know is
the thin solid crust at the Earth that we stand on,
to the underlying liquid mantle. If Earth's interior was made
up of the same material all the way through, the
waves should have been proportional to their distance. In other words,

(07:01):
they should have moved gradually slower as they got further
away from the quake's epicenter. But instead they started traveling faster.
The seismic waves are refracted faster through denser materials, so
researchers realized that those faster moving waves must have encountered
a denser layer of Earth. This was first observed by

(07:22):
a Croatian meteorologist and seismologist by the name of Andrea
Mohorovicic in nineteen oh nine, who went on to publish
his hypothesis about there being a discernible dividing line between
Earth's crust and mantle a decade later. This boundary zone
is known today as the Mohorovicic discontinuity or the MOHO

(07:43):
It's what the American drilling team named Project Mohole after.
Throughout the nineteen fifties and sixties, another earth shattering hypothesis
was emerging. The idea of plate tectonics. The more general
theory that the continents are aren't fixed, but rather shift
over eons of time, went back to nineteen twelve, but

(08:05):
it wasn't until nineteen fifty six, during a survey of
the Pacific Ocean floor, that scientists observed real evidence of
this in the form of on opposite sides of a
ridge on the seafloor symmetrical magnetic reversals, meaning that both
sides had been formed at the same time from the
ridge line and then spread apart from each other. So

(08:29):
when Project Mohole and the Cola Borehole began work, digging
deep was an exciting field of study and a flex
of a country's capacity with tech and engineering, similar to
the space race. We knew even then that the Earth's
crust must be relatively thin. On average, we think it's
about twenty five miles thick. That's forty kilometers, though it

(08:51):
tends to be thinner under the ocean floor and perhaps
obviously is thicker when you come across a mountain range.
The Cola team was hoping to dig down a little
over a third of that, but got stuck a little
under a third. The next layer down the mantle is
probably the thickest layer. It should continue for another one thousand,

(09:11):
eight hundred miles or three thousand kilometers, and seems to
consist of hot, dense rock that flows like asphalt and
moves the plates of the crust around. Underneath that, we
think that there is the liquid metal outer core, which
descends another one thousand, four hundred miles or two thousand,
two hundred kilometers or so, and then the solid metal

(09:33):
inner core, a hot dense ball of mostly iron with
a radius of about eight hundred miles or one thousand,
two hundred kilometers. So while the super Deep Borehole is impressive,
it only penetrates less than a third of Earth's crust
and just zero point two percent of the entire distance
to the center of the Earth. And until we actually

(09:56):
physically interact with these layers, we won't know for sure
what they're made up above in what conditions those materials
are under. Other attempts have been made through the years
by other countries, including Germany, Austria, and Sweden, and a
couple of oil wells have gone deep too, including one
in the Persian Gulf and one off the southeast coast

(10:17):
of Russia. None of those are deeper than the Kolo
super Deep Borehole, though some are longer having veered off
of their vertical courses. So we still haven't come anywhere
close to reaching the Moharrevichic discontinuity and finding out whether
we're right about the deeper layers of the Earth. Still,
scientists discovered a lot from the Kola Boorhole and the

(10:40):
other projects, as well a lot of today's drilling technology
in use by both scientific teams and petroleum Companies, was
first developed on the fly for Project Mohol or the
German Continental Deep Drilling program. The Kola borhole taught us
that a few of our hypotheses about the crust were wrong,

(11:00):
like how quickly the temperature rises, and also that there's
no conrad discontinuity, a transition boundary from granite rock to
basalt that geologists had reasoned to exist based on results
of seismic reflection surveys. The team also found liquid water
far deeper than we previously thought could exist, Harms said.

(11:21):
One of the unexpected results was certainly the occurrence of open,
saline water filled cracks, documenting that the crust is not dense,
but that pathways exist, allowing fluids to flow. Even more
exciting was the discovery of biological activity among the rocks.
At about four and a half miles down that's seven kilometers,

(11:43):
the team found dozens of fossils from single celled marine
organisms dating back two billion years. Some microscopic fossils, encased
and organic compounds were surprisingly intact despite the extreme pressures
and temperatures of the surrounding rock. But can we dig deeper?

(12:04):
Probably eventually, but Harms said. Digging deeper than twelve kilometers
depends on two critical factors temperature and borehole stability, the
latter being dependent on stress strain and drilling fluid composition
and weight. That'll take some pretty technologically advanced equipment. Considering

(12:26):
that temperatures there are predicted to be as high as
five hundred degrees fahrenheit or two fifty celsius, and the
real pie in the sky, or rather pie in the
Earth would be reaching Earth's mantle, Harms said, we can
learn a lot about the mantle if we get access
through drilling. Earth scientists want access to the real institu

(12:48):
mantle to understand the nature of this boundary that is
still debated and from which we have no fresh samples
that contain information on how the crust and mantle interact,
how fluids and magma droplets escape from the mantle into
the crust and ultimately into our hydrosphere, and how they
feed the biosphere, or how matter escapes back into the mantle.

(13:10):
These grand circles of how our planet evolves remain enigmatic
along this boundary, and moho discontinuity is therefore a prime
objective of scientific exploration. But people are trying for it.
In the early two thousands, Japan began work on a
gigantic drilling ship called the chiqu A Japanese word for

(13:32):
the Earth, which is designed to pick up where Project
Mohol left off, going for the mantle through the relatively
thin oceanic crust. This ship and others like it, have
been part of several international science programs over the past
couple of decades, co funded by multiple governments. In twenty
twenty one, a Japanese team working with one of these,

(13:53):
the International Ocean Discovery Program, achieved the deepest yet drill
into the ocean's crust, coming in at four point nine
miles or write over eight kilometers. Funding from the United
States has since shifted away from scientific ventures like this again,
but the international community is boring on. We'll keep you

(14:14):
updated in the future. Today's episode is based on the
article why did the Russians seal up the deepest holl
in the world on how Stuffworks dot Com, written by
Jennifer Walker. Journey brain Stuff is production of iHeartRadio in
partnership with how Stuffworks dot Com and is produced by
Tyler Quain. For more podcasts my heart Radio, visit the

(14:35):
iHeartRadio app, Apple podcasts, or wherever you listen to your
favorite shows.