My book is set far, far into the future, in a widely settled solar system. An expedition is sent to Neptune, and the expeditioners find out that there are massive cave systems reaching deep into Neptune's mantle, which is not so much bigger than Earth's diameter. There is also water trapped down here, and the heat is okay (my science might be wrong on this). A project to colonize Neptune is devised, and it begins with sealing off the cave system, before using displacements pumps to remove a lot of the air down there. This way, they lower the pressure to habitable levels. They are enduring the pressures due to their space suits, but that isn't an option for long-term settlement. After depressurizing the caves, they fill them up with breathable air, extracting gases like oxygen and nitrogen from Neptune's atmosphere.
After that, they set up nutrition sources, creating a nutrition-wise diverse ecosystem within the waters of the cave system. Maybe the have some soil with them as well and start cultivating inside the caves. They have wind turbines outside drawing power from the immensely intense winds of Neptune.
Obviously, this is not a very fleshed out idea, but it doesn't need to be. My book is soft sci-fi, so my ideas only need to make sense on a surface level and be theoretically possible. Thing is, I'm wondering if this idea is completely nonsensical. I'm a bit confused as to the nature of Neptune; is everything that surrounds the mantle just a huge ass ocean? If so, how does this plan work then? I mean, the wind turbines could just be replaced by a water turbine. Instead of extracting water from within the caves, they could just open up a hatch in the roof and let the water in from above. And is the temperature down there livable?