X-COM: Terror From the Deep

Turn-based tactical game where you command Earth's underwater defense force against an alien invasion from the ocean floor. Aliens dormant in the deep have been activated by signals from space, forcing you to fight in submerged environments instead of land-based missions. You'll manage a base, research captured alien tech, and deploy squads in grid-based combat against sea creatures and their advanced weapons. Missions take place on the ocean floor and inside alien structures. Between fights, you allocate resources, hire soldiers, and develop new equipment to counter the threat. The shift from air to water changes terrain tactics but keeps the core loop of base management and squad combat.