X-COM: Terror from the Deep is a computer game, the sequel to UFO: Enemy Unknown (or X-COM: UFO Defense in North America), and the second part of the X-COM series. It was developed and published by MicroProse. Its interface is more or less identical to its predecessor, as is its graphics technology. Terror from the Deep was recently released on Valve Software's Steam platform.
In the original game, the player was tasked with defending Earth from invasion by X-COM races from Mars. Now, many years after the first alien war was won, a new alien menace begins to emerge from the deep oceans. The game plays out under water, with base-building and combat all being submerged beneath the waves. The only time in fact that the game moves onto dry land is for terror site missions, when the aliens attack a port, ship, or island holiday resort.
X-COM: TFTD, like the first game, consists of two parts. The GeoScape world view, a 3D map of the world used to track and intercept alien craft, and manage base facilities, research, manufacture, finance, etc. through a series of menus and sub-screens. The BattleScape engine is used for close personal combat between squads of aliens and humans, and takes the form of a turn based, isometric view, in which you move and fight with the aliens.
Terror from the Deep has in many ways an even stronger atmosphere than the original X-COM: UFO Defense, due largely to the fact that it is set underwater, where visibility is reduced significantly, especially at night. This combined with the rich background detail of the aliens and excellent, complex, history of their societies, revealed to the player slowly via research, and the quality of the graphics, provide a very believable and sometimes terrifying experience for the player. Yet, it was flawed by an infamous bug in the research tree that could "block" mandatory research to finish the game.
Another possible inspiration is The Kraken Wakes by John Wyndham, which feature aliens as an underwater threat. It is also possible that some old deep-sea stories and movies like Twenty Thousand Leagues Under the Sea by Jules Verne had some influence on the design of the game, as some weapons and devices have a retro look.