An international team of scientists recently spent six weeks on China's Yangtze River looking for the endangered river dolphin. They were unable to find any, leading some to believe the animal has been driven to extinction.
The 8-foot-long mammal has lived in China's longest river for 20 million years. But massive increases in fishing, shipping and development have pushed the creature to the brink. The last confirmed sighting occurred in 2004, and the last captive dolphin died in 2002.
By definition, scientists don't consider an animal extinct until 50 years after the last sighting in the wild. And the recent survey focused on the river's main channel - it's possible some dolphins may yet hang on in the tributaries. But even if they do, it's unlikely the population can ever recover.