Developer: MicroProse
Release Date: June 1, 1995
Source Code Release Date: Not formally released
X-COM: Terror from the Deep is the sequel to X-COM: UFO Defense, with the aliens attacking Earth again, but this time they're invading the oceans, sending most of the game underwater for a change.
NOTE: If you have the data from both X-COM: UFO Defense/UFO: Enemy Unknown and X-COM: Terror from the Deep, you can use the "Mods" menu in the game to switch between the two.