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The BBC News article The Deepest Hole we have Ever dug says:

This is the Kola Superdeep Borehole, the deepest man made hole on Earth and deepest artificial point on Earth. The 40,230ft-deep (12.2km) construction is so deep that locals swear you can hear the screams of souls tortured in hell. It took the Soviets almost 20 years to drill this far, but the drill bit was still only about one-third of the way through the crust to the Earth’s mantle when the project came grinding to a halt in the chaos of post-Soviet Russia.

The first image below shows the top of the Kola Superdeep Borehole, and the caption says that it is welded shut.

Is there any way to form a reasonable hypothesis what would happen if it were opened today? Would the 12 km deep well have filled with water, or helium, or natural gas, or is it likely to be filled with air? Would it be under tremendous pressure on the other side of the bolted-down cap?

Kola Superdeep Borehole BBC

above: The borehole still exists - but the entrance has been welded shut (Credit: Rakot13/CC BY-SA 3.0), below: The borehole is located in the wilds of Russia's northern Kola Peninsula (Credit: Getty Images)

Kola Superdeep Borehole BBC

https://www.youtube.com/watch?v=Qu6KyDUKViY

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    $\begingroup$ I'm not sure what they've lined it with, but it's entirely likely that the bottom stretch of the hole has closed up due to plastic deformation. $\endgroup$
    – Spencer
    Commented May 6, 2019 at 13:57
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    $\begingroup$ It is difficult enough to keep an operating well open; that is why "high collapse" strength casing is often used. $\endgroup$
    – blacksmith37
    Commented May 6, 2019 at 16:26
  • $\begingroup$ A well this deep would have well over 5 strings of concentric casing. The outer one would be about 3 ft diameter $\endgroup$
    – blacksmith37
    Commented May 6, 2019 at 16:28
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    $\begingroup$ One could only guess; 0, 1, or 1 plus drill pipe; depending what was going on when they abandoned the hole. O = open hole with no casing yet run on the bottom . 1 = waiting for the next action , like cement to harden. And for a well that deep there would be "tieback" casings and others I don't understand. $\endgroup$
    – blacksmith37
    Commented May 8, 2019 at 18:24
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    $\begingroup$ If the Russians followed normal safe practice for abandonment ;They would have set a plug and pumped several hundred feet + of cement . It would be very risky to rely on a bolted or welded flange to seal a very deep well. $\endgroup$
    – blacksmith37
    Commented Mar 25, 2020 at 15:18

3 Answers 3

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During the drilling, the hole was filled with "drilling mud", which served to lubricate the bit, carry the cuttings to the surface and power the bit via a turbine.

When the well reaches a highest depth, there is a significant difference between the hydrostatic pressure of the mud column and the lithostatic pressure due to the weight of the rocks. As a result, the walls of the well can be destroyed (bend in), which leads to serious complications during drilling. In order to balance the rock pressure, there is an increase of density of the drilling mud to about 2 g per centimeter cubic.

Application of Superdeep Drilling Technology for Study of the Earth Crust, O V Savenok et al 2020

I expect they would not have gone to the expense of pumping out the mud (if even possible) as it would result in the hole caving in.

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    $\begingroup$ The hole very likely collapsed whether not not there was mud in it . Collapse resistance depends on the collapse strength of the casing. $\endgroup$
    – blacksmith37
    Commented Sep 3, 2021 at 16:18
  • $\begingroup$ You have to pump out the mud, and all the cuttings resulting from the borehole drilling, otherwise your bit will get stuck. Plus, it was a scientific borehole, so the goal was to perform some study at depth. In fact, mud and resulting cuttings were brought to the surface (see page 5 of this report iopscience.iop.org/article/10.1088/1755-1315/459/5/052066/pdf ) $\endgroup$
    – EarlGrey
    Commented Sep 5, 2021 at 8:52
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    $\begingroup$ @EarlGrey The mud and cuttings were brought up to the surface by pumping more (filtered) mud down the drill pipe. They would not be able to pump all the mud out of the hole (if that is what you are saying) with the equipment described. $\endgroup$
    – Keith McClary
    Commented Sep 5, 2021 at 16:28
  • $\begingroup$ You are right, in principle the mud at the bottom of the borehole would have a pressure of max ~240MPa (2g/cm3, times g times 12 km) ... unlikely to flush it out with N2 (or other gas) injection. $\endgroup$
    – EarlGrey
    Commented Sep 8, 2021 at 8:48
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Water has likely seeped in to the point of filling the hole up to sea level along with a high concentration of mineral deposits as a suspension of solids with the 180° Celsius bottom creating a lava lamp like movement of those solids as they cool near the surface. Also assuming they left the drill and shafts inside, a whole lot of rusting metal.

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    $\begingroup$ Welcome to Stack Exchange and thanks for your answer! I don't doubt that something like this is correct. In Stack Exchange it's important to include supporting links or sources for at least some aspects of answers (even though this answer didn't). Is it possible to add for example something about how you arrived at the 180° C figure? Thanks! $\endgroup$
    – uhoh
    Commented Mar 25, 2020 at 3:08
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    $\begingroup$ Not so much rust as there is no continuous source for oxygen. they would not have left the drill string ( unless a big problem) . But the casing strings will be roughly one inch thick (25 mm) thick. Drill strings are normally leased and returned. $\endgroup$
    – blacksmith37
    Commented Aug 28, 2021 at 20:33
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The only reasonable hypothesis is that it is filled with whatever it was filled with when the well was sealed.

A 12km long pipe with a sealed end is not going to have its contents replaced with something else easily. You could theorise that an intrusion of gas at the bottom opening of the pipe could expel liquid by replacement - possible, but slow.

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    $\begingroup$ I was hoping for something like giant ants from the center of the Earth, or red-hot magma. $\endgroup$
    – uhoh
    Commented May 6, 2019 at 13:17
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    $\begingroup$ Heh - I'm with you on this - I always dreamt it would be like opening a plug, and lava emptying from the bowels of the earth. $\endgroup$
    – Rory Alsop
    Commented May 6, 2019 at 13:19

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