[chris]

humans will never see a snowy bottom in our Bathyspheres.

Probably a good thing!

Taking the thought experiment a bit further on then,  if we did have a sufficiently deep ocean, would the underwater ice form up to a depth at which the pressure became to low to sustain it as a solid? So you would have a solid water ocean floor beyond a threshold depth?

[puppypower]

Water expands when it freezes into solid ice. This is why ice floats on liquid water. When we add pressure, we cause liquid water to contract, which can prevent the water from expanding into ice. Liquid water can exist down to about -55F by adding pressure because the pressure will not allow the water to expand into ice. At -55F the lack of expansion is compensated by the slow mobility of the water molecules, such that it no longer has the entropy of a liquid. A metastable solid can form.

This strange behavior is due to the unique properties of hydrogen bonding. Hydrogen bonding shows both covalent and polar character. As a polar bond, the EMF or electromagnetic force potential is lowered by the hydrogen bonds getting as close as possible; minimize charge potential. As a covalent bond, the hydrogen bond needs to expand, so the covalent bonding orbitals can overlap properly. Pressure keeps the hydrogen bonds from expanding, such that they remain more polar, unable to expand into the needed covalent orientations for ice.

This shift in hydrogen bonding between polar and covalent states is important to life. It allows high and low density water clusters to form in then continuous liquid water phase within life.  If the local water expands via a covalent dominate cluster, this can put the squeeze on a local enzyme, so it can't move property to function, If the cluster shifts more polar and thereby contracts, now the enzyme is loose and can react. Information moving through the water, via hydro bonding, has a physical consequence.

If we maintained pressure and continued to add heat,the heat will eventually cause the polar bonds to expand, allowing solid water to form at high temperatures and high pressures. If we continue to add temperature, eventually the hydrogen bonds start to break, such that the polar and covalent transition is moot. This water above its critical point. Hydrothermal water; above the critical point, is very corrosive to most minerals and grinds down organics to CO2.

If we continue to add even more temperature and pressure, water undergoes several additional phases changes into exotic phases. The first is superionic water, then ionic water, and then liquid metal water, and final solid metallic water, which has a density of about 3.5 grams/cc.

In metallic water, the oxygen atoms of water form the stationary matrix of the water metal. The hydrogen protons are able to move, similar to the way elections move in most metals. Along with the hydrogen protons, are mobile electrons. Metallic water is a metal composed of an oxygen matrix, that conducts electricity using a plasma of hydrogen protons and hydrogen atoms.

Metallic water can theoretically exist in the core of the earth. The conditions of the core are within the parameters of the metallic water phase. The hydrogen proton plasma current of liquid and metallic water would be very corrosive to the metallic iron in the core. The oxygen matrix can accommodate extra elections, with the plasma hydrogen fetching elections from the iron. The earth has large supply of natural internal energy, proportional to the amount of iron in the earth's core. The liquid and metallic water will attempt to amalgamate with the iron, with the hydrogen of metallic water putting irons elections into play. This is reflected in the slight negative charge of the oceans.

[wolfekeeper]

humans will never see a snowy bottom in our Bathyspheres.

Probably a good thing!

Taking the thought experiment a bit further on then,  if we did have a sufficiently deep ocean, would the underwater ice form up to a depth at which the pressure became to low to sustain it as a solid? So you would have a solid water ocean floor beyond a threshold depth?

Yes, that's what I meant, you'd get Ice VI forming at great depth, depending a bit on the temperature down there, and it would stay down there because Ice VI is denser than water at the same pressure and it needs the high pressure to stay solid.

Unfortunately

No, it's very fortunate! If the oceans froze from the bottom upwards,the climate would be very different and life would probably not have evolved.

That's true, but I'm talking about Ice VI which can only form and be maintained under very high pressure, but (unlike Ice V) appears at temperatures well above freezing. So you could end up, as Chris says, with a layer of Ice VI as the new sea floor but only at great depth where the pressure is high enough.