This is my first try at CIV VI, though I played previous versions. I'm into the early 20th Century. I haven't been at war with anybody in the past 100 turns. I have three destroyers, parked East, West, and South. Suddenly something appears next to my west destroyer and is promptly sunk by it; the next instant, the same sort of something appears next to my east destroyer and meets the same fate. I look around for any other enemy, for a civ at war with me, ANYTHING to explain this bizarre attack. No, nothing, nada. Can somebody tell me what just happened?