The old X-COM series games are definitely worth a re-play! If you have played the first game in the series (UFO Defense aka "XCOM: Enemy Unknown" in Europe) then this is pretty much the same deal, but in a different setting. Missions are now on land and under water, and there are more weapons to research and use. I loved the "terror" missions on large ships! However, I remember Terror From The Deep as sometimes excruciatingly difficult, as many others have pointed out. Also the graphics are more "gritty" than the first game, worse in my opinion.
