Underwater Bases were underwater habitats used by the military over long periods of time. They were especially in use during World War III.
(Note: The background section of this page uses most of the same words as Terra Futura's page on underwater hotels 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.
Tech Level: 11
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.