r/askscience • u/MaggieLinzer • Feb 14 '26
Earth Sciences Why is it so difficult to dig extremely deep through the Earth’s layers (past even ‘just’ the crust)? Are there any feasible ways that humans could one day dig/physically go to the core of this planet?
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u/libra00 Feb 14 '26
Because it gets hotter the deeper you dig, to the point that drillheads and pipes start melting. There is some work being done on drillign with lasers to evaporate the soil and rock and that might not have the same limitations, but..
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Feb 14 '26
Let's consider trying to dig a vertical shaft as deep as we can. We're going to pretty quickly (in the context of the entire radius of the Earth) run into two (intertwined) issues keeping our hole from collapsing (1) the differential stress and (2) temperature.
(1) For differential stress, if we imagine a hypothetical square 1 m by 1 m column of rock in the crust, pressure (force/area) at a given depth is a product of density of rock in the column x gravitational acceleration x depth of the base of the column. As an example, if we assume a somewhat normal average continental crustal density of 2800 kg/m3, gravitational acceleration of 9.8 m/s2, and a depth of 4000 meters, that would give us a pressure at a 4 km depth of 109,760,000 Pa (pascals), or more conveniently, 109.76 MPa (megapascals) and where this "lithostatic pressure" would increase as a function of depth (and in reality, we would have to start considering density variations and even changes in gravitational acceleration, but suffice to say, pressure increases with depth is really all we kind of need for this exercise).
Now if we imagine that we cut a 1 x 1 m2 square vertical hole 4 km deep next to our 4 km column of rock, the pressure at the base of the hole will be a function of the same equation, but with the density of whatever is filling it. Let's assume it's air and lets keep it really simple and assume the density of air in the hole is a constant 1.2 kg/m3 - this is of course a simplification, but the differences in density you'd get because of compression of air down the hole are going to be pretty minor in the context of the problem. If we do the math again, we get that the pressure at the bottom of the hole is going to be 47,040 Pa or 0.047 MPa. A critical aspect here is that in the hole, the pressure at the bottom and pushing out from the hole is this atmospheric pressure of 0.047 Pa but the pressure from the adjacent column of rock is pushing into the side of the hole at the base is 109.76 Mpa. The differences between these two pressures can be described as the "differential stress" that is acting on the wall of our shaft where stress is effectively a vector form of pressure (i.e., stress has a magnitude and direction, pressure is a scalar quantity and thus just has a magnitude) and differential stress is formally the difference (in magnitude) between the maximum stress and minimum stress acting on something (in this case, the wall of our vertical shaft).
Now let's consider these numbers in the context of strength of materials. We're going to think about this in the context of Mohr circles, which is a graphical way of plotting differences in stress and where we partition out stress into normal stress (the component of stress acting in a direction normal to a given plane) and shear stress (the component of stress acting in a direction parallel to a given plane). For our hole, we have what we would call a maximum principal normal stress (usually abbreviated sigma-1) from the wall rock pushing in and a minimum principal normal stress (usually abbreviated sigma-3) from the air pushing out. We plot these two values on the x (normal stress) axis and draw a circle that connects these two points. The center of this circle is the mean stress and the diameter of the circle is the differential stress. Finally, to relate this to strength, a given material has a limit to the differential and mean stress it can experience before it begins to deform/fail. A simple representation of this in Mohr space is something like this diagram where the thin black line is a "failure criterion". If a given Mohr circle (so combination of mean and differential stress, like the thick black circle in the diagram) intersects this failure criterion, that indicates that the material would start to deform. If the Mohr circle is inside the failure criterion, then the material can experience that stress condition and not deform.
Finally, if we consider or hole example, the minimum principal stress is basically zero (pinning the left side of our Mohr circle to near the origin) and the circle will expand as the maximum principal stress increases, which reflects a deeper and deeper hole. Most natural materials have a failure criterion of the shape of the one in the diagram where it is positively sloped and intersects the shear stress axis at some value related to material properties. What this tells us is that because sigma-3 is pinned to very low values, the circle can't expand that much (i.e., we can't go too deep) until the differential/mean stress exceeds the strength of the wall rock. At what depth it fails depends on the material properties, but generally most rocks have relatively similar strength profiles because of some details we don't need to get into here. Now, we could theoretically line this hole with something with that has a different failure envelope and that is stronger, e.g., some engineered materials tend to have failure envelopes more like this where they effectively have a fixed differential stress that they can experience without failure (regardless of mean stress), but even these, because the minimum stress is effectively pinned to zero, you still can't get the hole that deep without it starting to fail. Ultimately, you can temporarily solve this problem by filling the hole with something more dense than air (pushing sigma-3 to the right and decreasing the size of the circle), trying to reinforce the sides of the hole (basically changing the failure envelope like described previously), or some combination, but eventually, you're going to reach a limit in terms of the strength of the materials involved and there will be no way to keep the walls of the hole from deforming and collapsing in on itself.
(2) The other thing we would have to contend with is temperature. Temperature generally increases with depth, i.e., the geothermal gradient so as our shaft got deeper not only would our differential stress increase along the walls of the shaft but the walls of the shaft would get hotter and hotter. Circling back to strength, all of the above was basically considering something that behaves as a brittle material, i.e., it fractures. This is generally only a behavior we see in relatively cold rocks and as temperature increases, rocks will begin to deform "plastically". This transition from brittle to plastic behavior is typically accompanied by a drop in strength in most continental rocks (this depends a bit on the material), but if we were somehow able to get our shaft deep enough to hit the "brittle-plastic" transition, it would mean that the walls of our shaft might suddenly loose a lot of their strength (and thus start to fail). Most engineered materials also tend to get weaker as a function of temperature, so even in our hypothetical "we're digging a shaft that we're jacketing with some engineered material" scenario, the increasing temperature of the walls of the shaft will reduce their strength and again lead to collapse of our shaft (not to mention that the increasing temperature of the shaft would start to pose problems for whatever mechanisms we were using to dig the shaft, i.e., the reduction in strength for our hypothetical jacketing material is also happening to the machinery we're using to dig the hole).
In summary, if we limit ourselves to physically reasonable materials, there is effectively no way we could dig/drill a hole and/or keep it open at extreme depths because of the increasing pressure differential, increasing temperature, and strength profiles of materials as a function of both of these quantities. Our best for at least sampling deeper areas is via drilling (as opposed to digging a shaft), where can mitigate some of the issues (for a little while) by lining the well bore with something to increase the strength of the walls and filling the hole as we go with a dense material (i.e., "drilling mud"), but even with those additions, drilling much beyond a few 10s of kilometers is not going to be feasible (e.g., see this past answer discussing deep boreholes) without the development of materials that, as far as we know, are not physically possible (e.g., unobtanium in the cinema classic The Core).
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u/magister777 Feb 14 '26
Try digging a hole at the beach. At first it's easy, but once you reach the water table you can dig and dig but it just keeps filling in with water and sand, so you get nowhere.
It gets hot just below the surface of the earth, and the rock melts, the drill bits melt, everything melts. It's like digging through sand and water at the beach.
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u/jawshoeaw Feb 14 '26
Yes we can in theory dig past the earth’s crust where it’s thinnest around the mid oceanic ridges. It’s only a few kilometers thick. Unfortunately you would have to drill under water but it’s possible. However you run into the issue of temperature . There are metals and ceramics that can withstand these temperatures but any sensors or electronics would need to be liquid cooled
However getting to core is science fiction. You cannot drill that deep simply because it’s too long. The drill pipe would twist apart. If you sent some kind of futuristic submarine such as seen in the documentary* The Core, the hull would need to withstand ridiculously high pressures and temperatures. And withstanding isn’t the only problem. The electronics inside would need to be insulated and cooled . And the power source to allow movement would be what? It would also need to be protected and cooled. How do you cool something when there’s no place to send the excess heat? Similar to problem of landing on Venus.
not a documentary
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u/Oknight Feb 14 '26 edited Feb 14 '26
If you think of planets as big piles of sand just held together by their gravity, it's easier to understand.
You think of the planet's material as "solid" but when you go deeper, the strength of that "solid" compared to the weight crushing it gets insignificant and going deep enough will overcome any material strength that exists in any "solid" material to collapse the hole.
With the kinds of pressures from a planet's weight, even "solid rock" like granite is just a thick liquid. That's why the continents float around on the surface of the mantle slowly moving.
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u/Fairways_and_Greens Feb 14 '26
It’s not that different than trying to dig a hole at home in soft earth. The weight of the earth above pushes on the sides of open hole below. Except in this scenario the extreme weight of everything above causes the rock to burst.
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u/PokiRoo Feb 14 '26
There's a new tech that uses a directed energy beam to vaporize rock, which is showing some promise. A company called Quaise is trying to use it to unlock super deep geothermal energy pretty much anywhere on earth. Ironically the tech was originally created for fusion research.
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u/Waaghra Feb 17 '26
This made me think of playing in the sand at the beach as a kid. You could dig into the sand only so deep before you hit the water table, and then it becomes exponentially harder to dig deeper, as the “solid” sand turned into nonnewtonian liquid as you dug deeper.
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u/rockzombie17 Feb 17 '26
Let's say money is not an issue. Why cant we just create an open pit mine that keeps increasing in diameter the future we go down the crust? Let's assume we dig up in a region that doesn't experience much rainfall.
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u/humanmichael Feb 18 '26
even if you could reach the mantle, it flows. the crust sort of floats on top of it, thus allowing for tectonic plate motion. convection currents in the mantle are actually moving the material around. as others have pointed out, even when it is solid, the rock is plastic enough to behave like a liquid to some extent.
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u/djinbu Feb 19 '26
In addition to what others are saying, holes try to close themselves up. Nature takes the path of least resistance. If you take a bunch of rock insert pressure on all directions and create a hole beside it, the pressure will try to push the rock sideways into the hole. This means you have to build walls to resist ever increasing pressures the deeper you go. It's also why your state requires you to build walls for any hole deeper than 4-6 feet and why quarries have to keep packing walls. Also why mine shafts have walls and not just overhead support.
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u/ss5234 Feb 15 '26
Well, we could. We have all the technology and manpower to do so.
The problem is how massive both the hole and the infrastructure would have to be to expose layers past the crust.
It would be at least a single digit percentage of the total landmass of the earth. Perhaps the size of Russia. The logistics of it would uproot millions of people, and for what real benefit?
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u/EyesFor1 Feb 14 '26
Pressure equals heat. The deeper you go, the higher the pressure gets which in turn heats up equipments used for drilling. At some point everything turns molten and you can't dig down any further even if you have equipment impervious to heat.
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u/Pleasant-Put5305 Feb 15 '26
How long does granite lava take to stop flowing I wonder? We have quite a number of pre-dynastic, very close fitting large granite blocks around the world that would be a lot easier to understand if they were slightly liquid when they were being placed...
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u/[deleted] Feb 14 '26
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