It was a tragedy for the whales, but a gift for science: a mother and son (calf) of the world's least-known cetacean species, (Mesoplodon traversii) or the spade-toothed beaked whale, died in 2010 after stranding on a New Zealand beach. They were presumed to be a common species until a new DNA study pegged them as a species previously known from 3 skulls. It's a good reminder that the oceans are still vast, and they can shield even a large animal from all but chance detection.
It's not at all certain, though, that this is the least-known whale. Cetologists I talked to in the writing of my 2006 Shadows of Existence thought there was at least one type, nicknamed Mesoplodon species B, that had never been caught or its remains found, and there may be a couple more after that. (That is, of course, if you accept the identification of the long-enigmatic Mesoplodon species A with the pygmy/Peruvian beaked whale. I admit I have not read the latest research, but that wasn't very convincing to me.)