In 2011 a Swedish salvage team scanning the Gulf of Bothnia floor got a sonar return of a large circular object with what looked like ramps and right angles, and the internet promptly decided they had found a crashed flying saucer. Geologists who examined the samples the divers brought up identified glacial rock, most likely a moraine deposit dropped by the retreating ice sheet. A good reminder that the sea floor is genuinely unexplored and that most anomalies are geology. Told honestly.
The northern Baltic Sea (Swedish waters)
See it on the map