X-COM: Terror from the Deep

X-COM: Terror from the Deep takes place mostly underwater, with base-building and combat all being submerged beneath the waves. This is also used as a plot device; all of the alien technology from the first game is unusable in salt water, forcing the player to capture and develop new technology.

Just like UFO: Enemy Unknown, the game consists of two parts. The first is the real time-based GeoScape, a global view of Earth where the player views alien and X-COM craft and bases, can hire and dismiss staff, buy and sell vehicles, weaponry, ammunition and items, and build and expand bases. The second part, the BattleScape, is used for combat between squads of aliens and humans, and takes the form of a turn-based battle from an isometric view.

Terror from the Deep is set in 2040, four decades after the events of UFO: Enemy Unknown. Following the destruction of the alien Brain on Cydonia, a transmitter remained active there which awakened a group of aliens under the Earth's seas who had lain dormant for millions of years. After awakening, the aliens proceed to terrorize seagoing vessels and port cities, kidnapping humans to perform bizarre genetic experiments on them. X-COM, which had been disbanded after the first alien war, is revived by the Earth's governments to fight this new menace as the aliens' ultimate goal is to reawaken their supreme leader, a being that cannot be stopped once revived.

Eventually, it is revealed the aquatic aliens, cousins of the Sectoids from Enemy Unknown, came to Earth on a massive spacecraft, known as T'Leth, that crashed into what is now the Gulf of Mexico 65 million years ago, causing the extinction of the dinosaurs. Destruction of T'Leth by the player results in victory but also accidentally results in another worldwide environmental cataclysm, destroying the ecosystem of Earth and setting the stage for the third game in the series, X-COM: Apocalypse.

MicroProse wanted the developer Mythos Games to make a sequel to UFO: Enemy Unknown in six months. Julian Gollop felt that the only way to do so was to change the graphics and make minor changes to the gameplay. Eventually, MicroProse licensed Mythos' code and their internal UK studio created Terror from the Deep within a year, while Mythos Games began developing the next sequel, Apocalypse.

MicroProse artist Terry Greer recalled: "A decision was made to use the original engine, reskin the graphics and create a whole new story. By keeping changes to the absolute minimum a sequel could be created in just a few months. Also, by not inventing any new game features or game technology it would make the scheduling one largely led purely by asset creation – which makes it whole lot easier when it comes to estimating task durations and scheduling." Simultaneously with making the console port of Terror, MicroProse UK also began work on their X-COM: Alliance project.

Together with its predecessor, UFO: Enemy Unknown, Terror from the Deep's sales had passed 1 million copies by March 1997. The game was received mostly very well by critics. PC Gamer UK called it to be "not only a great sequel to UFO but a superb game in its own right." Computer Game Review offered a rave review and awarded the game a "Platinum Triad" score.

On the other hand, GameSpot stated that "apart from new art and a handful of new combat options, this is exactly the same game as UFO Defense, only much more difficult." Next Generation felt the same: "In the end, X-COM 2 is still a phenomenal game, and has been scored as such, but it seems so much more could have been accomplished to make this game more than just a carbon copy of an old game."