The Baiji river dolphin, which once swam extensively throughout the Yangtze River system, became the first cetacean species to be declared functionally extinct in modern times after an extensive six-week survey of the animal's habitat in 2006 turned up no sign of the species.
Then, just a year later, a Chinese man videotaped a dolphin gleefully leaping from the surface of the Yangtze, which experts later confirmed was indeed a Baiji dolphin. It's impossible to say how many of these dolphins still swim in the river, but it's certainly possible, given the rarity of the sighting, that this dolphin was the last of its kind.
Unfortunately, even if several individuals still cling to existence, experts says the species should still be considered functionally extinct, since such a small population would not be genetically viable. But so long as sightings occur, there's hope.