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In 1970, the USSR began drilling at the Kola Super-deep Borehole. The target depth was 15,000 meters;

The stated areas of study were the deep structure of the Baltic Shield, seismic discontinuities and the thermal regime in the Earth's crust, the physical and chemical composition of the deep crust and the transition from upper to lower crust, lithospheric geophysics, and to create and develop technologies for deep geophysical study,

according to its Wikipedia entry.

Data obtained from previous seismic experiments had been interpreted to indicate a transition, which was supposed to be from granite above to basalt below about 7,000 meters. However, one of the surprising findings of the experiment was that there was no such change of kind of rock; the difference was metamorphic, now said to be a result of the changing pressure. Also,

...the rock at that depth had been thoroughly fractured and was saturated with water, which was surprising. This water, unlike surface water, must have come from deep-crust minerals and had been unable to reach the surface because of a layer of impermeable rock.

The source for this entry is Allen Bellows at damninteresting.com (which coincidentally places the supposed granite-basalt transition at "3-6 km"):

Because free water should not be found at those depths, scientists theorize that the water is comprised of hydrogen and oxygen atoms which were squeezed out of the surrounding rocks due to the incredible pressure.

So, why should water not be found at those depths? Based on what assumptions were people surprised to find water there? Or was it only journalists who were surprised?

I'm sorry I don't have more initial research; the truth is I don't know where to start looking. Online searches only give me results like this article at Science Magazine, which is related only because in its case, too, water is supposed to be "squeezed out" of rock, synthesized, as it were from its chemical components. (I asked this question at Codidact.physics, but it seems to need the broader context of geology)

Related: What do continents "lay" on?

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    $\begingroup$ There is , in oil and gas provinces . I can't say it is connate or in solution with hydrocarbons , but it causes corrosion problems. Search SPE references for HPHT wells ( high pressure, high temperature ). Of course the Kola well was based on oilwell techniques and equipment. $\endgroup$ Apr 7 at 16:09
  • $\begingroup$ A recent paper (discussed and linked here) says that the mantle is absorbing water as it cools. $\endgroup$ Apr 7 at 20:14
  • $\begingroup$ Well to start with because the rock was 180 degrees C, well above the boiling point of water. $\endgroup$
    – John
    Apr 12 at 1:27
  • $\begingroup$ @john Yes, the boiling point at sea level. But it's not the same at other pressures, is it? $\endgroup$
    – Conrado
    Apr 12 at 1:52
  • $\begingroup$ @Conrado it does change but it does not change that much with just a few kilometers. $\endgroup$
    – John
    Apr 12 at 5:25
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There are two reasons for this surprising finding:

  1. Compaction. Once you have high pressures at depth, the less dense material goes up. Take a bucket of water, put sand in it, and squeeze the sand really hard. Water goes up. Then, you'd expect no water at depth of several kilometres.
  2. Even if you somehow had free water at these depths, the expectation is that they would form hydrous but solid minerals. For example, adding water to diopside, enstatite, and quartz leads to the formation of hydrous, but solid, tremolite: $$\ce{2CaMgSi2O6 + 3MgSiO3 + SiO2 + H2O = Ca2Mg5Si8O22(OH)2}$$ or, adding water to quartz and corundum leads to the formation of kaolinite: $$\ce{Al2O3 + 2SiO2 + 2H2O = Al2Si2O5(OH)4}$$ Increasing pressure leads to the equilibrium in these reactions shifting to the products, i.e. the solid hydrous phases without any free water.

As to why the water was actually found there, there's still some debate in the scientific community. One of the explanations ("...must have come from deep-crust minerals and had been unable to reach the surface because of a layer of impermeable rock") is definitely reasonable. The other ("scientists theorize that the water is comprised of hydrogen and oxygen atoms which were squeezed out of the surrounding rocks due to the incredible pressure.") is probably incorrect.

This (paywalled) paper provides an overview in a very accessible language: https://doi.org/10.2138/gselements.16.6.381

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Nothing really prevents water from existing deep inside Earth. We have direct evidence that it does, even in the mantle.

Evidence for such water may be found in Ice VII inclusions in diamonds[1]. Because of their great mechanical strength, diamonds formed in the mantle retain GPa-level pressure within their lattice, so when they come to the surface and cool off any water trapped in them condenses into the high-pressure Ice VII phase instead of the ordinary Ice I$_{\text{h}}$ or liquid phase.

Such a phase with the formula $\text{H}_2\text{O}$ would have to come from uncombined water as distinct from hydroxyl groups in rocky matter. From the abstract:

The [Ice VII] inclusions suggest that local aqueous pockets form at the transition zone boundary [between the upper mantle and lower mantle] owing to the release of chemically bound water as rock cycles in and out of this region.

This answer from Space Exploration Stack Exchange describes the finding of diamond-borne Ice VII in more detail and suggests the possibility of finding other high-pressure ices in outer-planet moons through seismic measurements.

Reference

1. O. Tschauner, S. Huang, E. Greenberg, V. B. Prakapenka, C. Ma, G. R. Rossman, A. H. Shen, D. Zhang, M. Newville, A. Lanzirotti, K. Tait, "Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth’s deep mantle", Science 09 Mar 2018: 359, Issue 6380, pp. 1136-1139. DOI: 10.1126/science.aao3030.

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    $\begingroup$ Ok, but why not liquid water? I mean, they found liquid water (in the real experiment), but the wikipedia article (among others) implies that their model had predicted that there would be no liquid water. I can't understand how you would keep water out of the deep crust. Was the water on top of the impermeable layer supposed to have come from outer space? $\endgroup$
    – Conrado
    Apr 16 at 12:55
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    $\begingroup$ The quotation is "free water". They do not seem to say it was a liquid. Water deep inside Earth would exist in a supercritical state, possibly with extensive ion dissociation under sufficiently high temperature and pressure. $\endgroup$ Apr 16 at 13:41
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    $\begingroup$ I see, you're right--I was misunderstanding, because I don't really know my chemistry. But then, are they saying that free water shouldn't exist even in a supercritical state? Supercritical water is still water, isn't it? Or was it expected to be chemically un-maintainable as water in those conditions? $\endgroup$
    – Conrado
    Apr 16 at 13:52
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    $\begingroup$ It was something as a surprise when water was found deep inside the crust and mantle, which were supposed to be nothing but rock. But now it's becoming accepted, and even proposed to reach and react with Earth's core. $\endgroup$ Apr 16 at 14:01

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