Exotic planet idea.

[Spaceception]

Heyo everyone, while on a road trip yesterday, I had an idea for a weird but potentially awesome planet, and it follows as so;

The planet is however big it needs to be, but I imagine it'd be bigger than Earth.

What makes this planet exotic, is that it has some highly dense atmosphere or dense fluid that water could sit on top of (Supercritical CO2? Idk), with either pockets, or an entire ocean of liquid water above that, and a regular, thinner (But extremely humid) atmosphere above the water.

I just woke up, so forgive me if this is completely stupid, but could it work? If it's impossible to form naturally, could it at least be possible with a sufficiently advanced alien society? Are elements of this possible? It's for Infinitum, so I'm fine with some inaccuracies.

[Zeiss Ikon]

As far as I know, there's no gas that's denser than liquid water at any pressure (not even supercritical CO2 -- though you might see if you can find a phase diagram with densities that covers supercritical radon).  There are "pools" at the bottom of Earth's ocean, in many locations, formed of water with increased density due to something dissolved (either hypersaline, or sea water with hydrogen sulfide, generally).

Even if you had a "gas" in pools below an ocean, it would behave like a liquid -- the density would mean you'd float in it, even more so than in water, and while its compressibility might cause some odd behaviors, you'd be at the limits of pressure tolerance for even remotely piloted machines to explore it...

[sevenperforce]

Liquid mercury, maybe.

[Scotius]

There you go:

https://en.wikipedia.org/wiki/Clathrate_hydrate

Gas under the water. Though it's more like wax than free-floating bubbles

[Zeiss Ikon]

5 hours ago, Scotius said:

There you go:

https://en.wikipedia.org/wiki/Clathrate_hydrate

Gas under the water. Though it's more like wax than free-floating bubbles

First sentence of the linked Wikipedia article: "Clathrate hydrates, or gas clathrates, gas hydrates, clathrates, hydrates, etc., are crystalline water-based solids physically resembling ice, in which small non-polar molecules (typically gases) or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen bonded, frozen water molecules."

That's a solid, not a gas.  And unless the trapped gas molecules increase the density significantly above that of ice, it'll float, not sink.