Second Alien War

AW-II was a war fought between X-COM forces and a number of aquatic alien races beneath the surface of Earth's oceans. These aliens, apparently in cryo-stasis for thousands of years, were awakened by an automated signal sent from the alien base at Cydonia at the end of the First Alien War in 2002. The signal activated the alien city of T'leth, hidden in the Sigsbee Deep in the Gulf of Mexico, unleashing the horde of aquatic aliens therein. Fighting a long and arduous battle, X-COM finally located and destroyed T'leth. The eradication of the alien city ended the invasion, but with grave results. The resulting cataclysmic explosion heavily contaminated Earth's atmosphere, wreaking ecological havoc that continues to plague Earth to this day.