Thought experiment: a sealed container with water in it, along with a balloon. You are able to control the balloon with a string from the outside somehow, and this string however is still sealing perfectly. No leaks in the system. The balloon is taken down to the very bottom of the container by you. You expended some energy to go against the buoyant force. When the balloon got to the bottom of the container, what happens? Normally a balloon would shrink due to hydrostatic pressure. However in this case, the balloon cannot shrink because it would have to suck water into its place where it was taking up space before. It cannot suck water in to place because the container is sealed tight without any air in it. Would this put the whole container under vacuum? If it did, then the hydrostatic pressure would just press out that vacuum, no? The balloon stays the same size? Can this be the case even though the balloon is very sensitive to pressure changes in typical settings? And if I had the ability to jump inside of the balloon, what would be inside? Just regular air pressure trapped inside not affected at all by hydrostatic pressure?