At the time the Earth was split into two power blocs ready to unleash nuclear and proton missiles against each other. In order to make the war as devastating as possible, it was originally decided to send up a number of satellites into orbit above the Earth. But when it was proven that space stations were vulnerable, it was decided to construct a series of sea bases which contained the majority of the different blocs' arsenals.
The Sea Bases were controlled from their headquarters Sea Base Command, who ordered the respective commanders to maintain radio silence at all times. Regularly each base would go through missile run tests to prepare their synch-operators for the real war. These practices could be triggered by outside events or chosen at random by the computer.
Sea Bases were heavily armed with proton missiles and with energy traces. They featured a number of airlocks with heavy bulkhead doors.
In 2084, the Fifth Doctor, Tegan and Turlough arrived at Sea Base 4 after the TARDIS was severely damaged in orbit when they had run into a satellite weapon. They quickly became involved in an attack by joint Silurian-Sea Devil operation to commandeer the bases' proton missiles to begin the war, allowing the Silurians and the Sea Devils to retake the Earth. (TV: Warriors of the Deep)
Other references Edit
After materialising aboard a base, the Tenth Doctor suggested to Rose Tyler that they had arrived at a sea base before he soon realised that they were in a sanctuary base, Sanctuary Base 6. (TV: The Impossible Planet)