You encounter many strange things at sea. Once during my night watch, when we were hundreds of miles from anywhere, the sonar kept going off indicating that the "bottom" was only 13 meters below, then 15 meters, then 12, then 17. Very strange. I told Uncle Kurt, who just smiled and said, "Yep, something's probably swimming below us." It was probably a whale, but I liked to picture something hideous and tentacled and extremely hungry that had ventured up from the Tongan trench (which is a part of the ocean that at it's deepest is over 10,000 meters deep).
Another strange thing was marked on the chart as a "magnetic anomaly". This was a largish patch of ocean we sailed through between Tonga and New Zealand where our compass got all confused and pointed in the wrong direction. Fortunately for us, the GPS, was working just fine. I'm not sure what caused this--an enormous metal deposit on the ocean floor?