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

This is the Kola Superdeep Borehole, the deepest manmade 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

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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 May 6 '19 at 13:57
  • $\begingroup$ It is difficult enough to keep an operating well open; that is why "high collapse" strength casing is often used. $\endgroup$ – blacksmith37 May 6 '19 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 May 6 '19 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 May 8 '19 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 Mar 25 at 15:18
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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 May 6 '19 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 May 6 '19 at 13:19
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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 Mar 25 at 3:08

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