I always like coming across the little nuggets that remind us we don't know everything about the natural world. Dr. Bruce Robison of the famed Monterey Bay Aquarium Research Institute once told an interviewer he thought we had so far missed at least a third of the large animals in the oceans (though he didn't define "large.")
In the fascinating new book Harpoon: Into the Heart of Whaling (Da Capo Press, 2008) Andrew Darby chronicles the long and often bloody interactions between humans and whales. He also includes this item in his discussion of the now-closed whaling center on King George Sound in Albany, Western Australia. In the 1970s, when whalers like Captain George Cruikshank followed sperm whales with sonar, they often watched something they coulnd't identify in the 4000-meter-deep Albany Canyon.
"'The mystery bugger,' they called it...They would pick it up always at the same spot, and follow it by sonar as it cruised through the canyon, leaving a larger imprint on the screen than a whale. The creature would almost break the surface and then disappear. It was no air breather. It might have been a giant squid, or a giant shark. They never found out." (p.96)