X-COM: Terror from the Deep is a strategy video game developed and published by MicroProse in 1995 for the PC, with ports for the 3DO Interactive Multiplayer and Sony PlayStation in 1996. It is the sequel to UFO: Enemy Unknown (aka X-COM: UFO Defense) and the second game of the X-COM series.
The game is set in 2040, decades after the first Alien War was won, when a new alien menace begins to emerge from the oceans. Eventually, it is revealed the aquatic aliens 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.
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.
Underwater battles use the same physics as the ground ones. The game features some mission types composing of multiple parts, such as alien shipping route terror attacks in which the first part is a battle of the upper floors of the ship while the second part takes places in the lower decks of the ship (all parts must be completed for these missions to be successful, and soldiers lost in previous parts do not appear in later parts).
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