Extreme Depth Habitats were underwater habitats that operated at extreme underwater depths. They were relativelly rare.
(Note: The background section of this page uses most of the same words as Terra Futura's page on underwater colonies to save time.)
The first underwater habitats were constructed in the 1960s. Jacques Cousteau, who invented scuba gear, built many of these. Most were open pressure. Some were closed pressure. There were even some that were both. Most underwater habitats during the Cold War were outposts designed to diving, underwater construction, studying aquatic life, training dolphins, military research and espionage, maintenance, construction of oil rigs, climate research, tourism, aquaculture, and training for astronauts. They were built on the surface and placed into the sea by submersibles. They were still connected to the surface for power and communications. As time went on, underwater habitats became less dependent on the surface. They started providing accommodations for people. This gave rise to underwater hotels.
The first underwater hotel to be built was the Jules Underwater Lodge in the Florida Keys. It had five rooms and could only accommodate six passengers. People had to scuba dive to get there. The Poseidon Undersea Resort went many step ahead of the Jules Underwater Lodge. Built in Fiji in 2014, it was connected to the surface by tunnels. Each suite had a view of the coral reef. They also had bathtubs and sinks. Finally, they had exterior lights and fish-feeders. There was also a rotating restaurant. Insulation was incorporated into the transparent windows to keep heat from escaping. The Hydropolis Hotel in Dubai was built with a land station connected to the main hotel via tramway. Other underwater hotels were also built. But during World War III, the military needed underwater habitats that could last for weeks. This led to underwater bases.
In The Transformers, the Decepticons operate on Earth using an underwater base built from their own ship. During World War III, humans also developed the same technology. Like the underwater outposts of the Cold War, underwater bases of World War III were hidden from view by water. They could hold more personnel for weeks, if not months. Modules were constructed on the surface and then lowered into the sea by AUVs. They had a combined open pressure/closed pressure system to make conditions more comfortable. These large bases still had connections to the surface but were more self-sufficient in case these failed. Artificial gills were used to supplement power and air in that case. After World War III, underwater habitats were less utilitarian and more residences. The first underwater cities were built.
When people started colonizing the sea, underwater cities were built to accommodate the colonists. Some were built around artificial islands. Other were the result of subsea aquaculture. There were even those that were built with military advantages in mind. Others were coastal cities that expanded into the sea. Underwater cities were far more self-sufficient than earlier underwater habitats. Artificial gills were used to create breathable oxygen. Desalination created drinking water. Underwater cities even had their own power sources while still connected to the surface. Underwater cities were almost entirely closed pressure systems. Underwater cities had some structure underground. Over time, underwater cities gave rise to underwater colonies.
Underwater colonies were more advanced than underwater cities in that they were completely self-sufficient. Of course, they still had contact with the surface. The traded with the surface and had tourists. They made their own food and drinking water through aquaculture and desalination, respectively. Air was provided by artificial gills. Power was provided by fusion reactors. Underwater colonies also had their own industries and recycling. Advances in technology allowed the long-term establishment of closed pressure habitats as deep as 250 meters. In the future, underwater habitats went even deeper.
Tech Level: 13
As time went on, there were underwater habitats even at the lowest depths of the ocean. These were not very large. Usually, humans only went down there to supervise robots. The purposes for such extreme depth habitats included mining, exploration, and scientific research. Some underwater colonies did exist at those depths. These were people who modified their DNA to be able survive at higher underwater pressures, to be anthropomophic marine animals, to be mermaids, or just to have gills.