Undersea habitation units are small stations located beneath the surface of the ocean in which a crew of aquanauts may live. The purpose of these structures may be research or maintenance of deep sea equipment such as oil rigs. They may contain laboratories, moon pools, airlocks, and vehicle bays. If large enough, they may be considered an underwater city. They are home to scientists, divers, and things that came in from the deep.

When designing undersea habitation units for a fictional world, consider the location in which they reside and the purpose for which they were constructed. For instance, if they are performing research, they may be located near a coral reef, a kelp forest, an oceanic trench, or some other terrain feature of interest. Keep in mind the depth at which the unit exists, as deeper waters will have greater pressures.

The structural design of the undersea habitation unit is also a crucial consideration. The unit must be designed to withstand the extreme pressure of the ocean at its specific depth. Rounded shapes tend to be more structurally sound than square or rectangular shapes, as they distribute pressure more evenly.

Life support systems are necessary for any undersea habitation unit. The systems must be designed to provide oxygen and remove carbon dioxide from the air. They must also filter water for consumption and remove waste. A backup system is also essential in case of emergencies. Some undersea habitation units are tethered to the surface. This tether provides air, power, and a line of communication. Other units are entirely self-sufficient.

A reliable power supply is critical for any undersea habitation unit. The unit will require electricity for lighting, heating, and other essential systems. In fictional worlds, deep sea vents may provide heat and power.

Communication is essential for any undersea habitation unit. The unit must have a reliable communication system to communicate with the outside world in case of emergencies. The system should also be able to receive weather and oceanic condition updates to ensure the safety of the inhabitants.

Living in an undersea habitation unit can be isolating, so recreational facilities are necessary to maintain the mental health and well-being of the inhabitants. These facilities can include a gym, a library, and entertainment systems.