When the vessel dives, it takes on liquid, increasing its mass but not its volume, thus making it heavier than the surrounding water. It now has negative buoyancy and will maintain this negative buoyancy until it sinks to the denser water below. When the sub surfaces, the process is reversed. It would take weeks for a nautilus to adjust its density enough to travel the distance that it does in only two hours. Contrary to earlier theory, the hydroponone itself is responsible for the daily migration, using a process of jet propulsion.
Water is sucked in and then shot out, and the nautilus goes in whatever direction it wants.
Steady at one atmosphere, the gas pressure in a nautilus's chambers has been found insufficient to bolster shell integrity at great depths. Although early scientists dismissed this possibility, it turns out that the shell's structure itself is strong enough to withstand depths of up to 2,000 feet without the aid of additional internal gas pressure. As a nautilus grows, it gradually drains liquid out of previously formed chambers to compensate for the added mass of the expanding shell and retain neutral buoyancy. For a full-grown nautilus, occasional minor adjustments to the amount of liquid in its chambers are made to compensate for large meals or chips in its shell. A nautilus tends to establish neutral buoyancy for its average depth, which is usually around 900 feet in the ocean.